feat(process): add Program struct with Find, Run, RunDir

Add Program to the process package as a lightweight tool-finder and runner.
Find() resolves a binary via exec.LookPath (wrapping ErrProgramNotFound),
Run() and RunDir() execute the binary and return trimmed combined output.
Includes 7 tests covering happy paths, error paths, and the errors.Is contract.

Co-Authored-By: Virgil <virgil@lethean.io>

docs/plans/2026-03-18-absorb-sail-program.md

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+# Plan: Absorb Sail Program
+
+**Date:** 2026-03-18
+**Branch:** agent/implement-the-plan-at-docs-plans-2026-03
+
+## Overview
+
+Add a `Program` struct to the `process` package that locates a named binary on PATH and provides lightweight run helpers. This absorbs the "sail program" pattern — a simple way to find and invoke a known CLI tool without wiring the full Core IPC machinery.
+
+## API
+
+```go
+// Program represents a named executable located on the system PATH.
+type Program struct {
+    Name string // binary name, e.g. "go", "node"
+    Path string // absolute path resolved by Find()
+}
+
+// Find resolves the program's absolute path via exec.LookPath.
+func (p *Program) Find() error
+
+// Run executes the program with args in the current working directory.
+// Returns combined stdout+stderr output and any error.
+func (p *Program) Run(ctx context.Context, args ...string) (string, error)
+
+// RunDir executes the program with args in dir.
+// Returns combined stdout+stderr output and any error.
+func (p *Program) RunDir(ctx context.Context, dir string, args ...string) (string, error)
+```
+
+## Tasks
+
+### Task 1: Implement Program in program.go
+
+- Create `program.go` in the root `process` package
+- Add `ErrProgramNotFound` sentinel error using `coreerr.E`
+- Add `Program` struct with exported `Name` and `Path` fields
+- Implement `Find() error` using `exec.LookPath`; if `Name` is empty return error
+- Implement `RunDir(ctx, dir, args...) (string, error)` using `exec.CommandContext`
+  - Capture combined stdout+stderr into a `bytes.Buffer`
+  - Set `cmd.Dir` if `dir` is non-empty
+  - Wrap run errors with `coreerr.E`
+  - Trim trailing whitespace from output
+- Implement `Run(ctx, args...) (string, error)` as `p.RunDir(ctx, "", args...)`
+- Commit: `feat(process): add Program struct`
+
+### Task 2: Write tests in program_test.go
+
+- Create `program_test.go` in the root `process` package
+- Test `Find()` succeeds for a binary that exists on PATH (`echo`)
+- Test `Find()` fails for a binary that does not exist
+- Test `Run()` executes and returns output
+- Test `RunDir()` runs in the specified directory (verify via `pwd` or `ls`)
+- Test `Run()` before `Find()` still works (falls back to `Name`)
+- Commit: `test(process): add Program tests`
+
+## Acceptance Criteria
+
+- `go test ./...` passes with zero failures
+- No `fmt.Errorf` or `errors.New` — only `coreerr.E`
+- `Program` is in the root `process` package (not exec subpackage)
+- `Run` delegates to `RunDir` — no duplication

program.go

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+package process
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"os/exec"
+	"strings"
+
+	coreerr "forge.lthn.ai/core/go-log"
+)
+
+// ErrProgramNotFound is returned when Find cannot locate the binary on PATH.
+// Callers may use errors.Is to detect this condition.
+var ErrProgramNotFound = coreerr.E("", "program: binary not found in PATH", nil)
+
+// Program represents a named executable located on the system PATH.
+// Create one with a Name, call Find to resolve its path, then Run or RunDir.
+type Program struct {
+	// Name is the binary name (e.g. "go", "node", "git").
+	Name string
+	// Path is the absolute path resolved by Find.
+	// If empty, Run and RunDir fall back to Name for OS PATH resolution.
+	Path string
+}
+
+// Find resolves the program's absolute path using exec.LookPath.
+// Returns ErrProgramNotFound (wrapped) if the binary is not on PATH.
+func (p *Program) Find() error {
+	if p.Name == "" {
+		return coreerr.E("Program.Find", "program name is empty", nil)
+	}
+	path, err := exec.LookPath(p.Name)
+	if err != nil {
+		return coreerr.E("Program.Find", fmt.Sprintf("%q: not found in PATH", p.Name), ErrProgramNotFound)
+	}
+	p.Path = path
+	return nil
+}
+
+// Run executes the program with args in the current working directory.
+// Returns trimmed combined stdout+stderr output and any error.
+func (p *Program) Run(ctx context.Context, args ...string) (string, error) {
+	return p.RunDir(ctx, "", args...)
+}
+
+// RunDir executes the program with args in dir.
+// Returns trimmed combined stdout+stderr output and any error.
+// If dir is empty, the process inherits the caller's working directory.
+func (p *Program) RunDir(ctx context.Context, dir string, args ...string) (string, error) {
+	binary := p.Path
+	if binary == "" {
+		binary = p.Name
+	}
+
+	var out bytes.Buffer
+	cmd := exec.CommandContext(ctx, binary, args...)
+	cmd.Stdout = &out
+	cmd.Stderr = &out
+	if dir != "" {
+		cmd.Dir = dir
+	}
+
+	if err := cmd.Run(); err != nil {
+		return strings.TrimSpace(out.String()), coreerr.E("Program.RunDir", fmt.Sprintf("%q: command failed", p.Name), err)
+	}
+	return strings.TrimSpace(out.String()), nil
+}

program_test.go

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+package process_test
+
+import (
+	"context"
+	"path/filepath"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	process "forge.lthn.ai/core/go-process"
+)
+
+func testCtx(t *testing.T) context.Context {
+	t.Helper()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	t.Cleanup(cancel)
+	return ctx
+}
+
+func TestProgram_Find_KnownBinary(t *testing.T) {
+	p := &process.Program{Name: "echo"}
+	require.NoError(t, p.Find())
+	assert.NotEmpty(t, p.Path)
+}
+
+func TestProgram_Find_UnknownBinary(t *testing.T) {
+	p := &process.Program{Name: "no-such-binary-xyzzy-42"}
+	err := p.Find()
+	require.Error(t, err)
+	assert.ErrorIs(t, err, process.ErrProgramNotFound)
+}
+
+func TestProgram_Find_EmptyName(t *testing.T) {
+	p := &process.Program{}
+	require.Error(t, p.Find())
+}
+
+func TestProgram_Run_ReturnsOutput(t *testing.T) {
+	p := &process.Program{Name: "echo"}
+	require.NoError(t, p.Find())
+
+	out, err := p.Run(testCtx(t), "hello")
+	require.NoError(t, err)
+	assert.Equal(t, "hello", out)
+}
+
+func TestProgram_Run_WithoutFind_FallsBackToName(t *testing.T) {
+	// Path is empty; RunDir should fall back to Name for OS PATH resolution.
+	p := &process.Program{Name: "echo"}
+
+	out, err := p.Run(testCtx(t), "fallback")
+	require.NoError(t, err)
+	assert.Equal(t, "fallback", out)
+}
+
+func TestProgram_RunDir_UsesDirectory(t *testing.T) {
+	p := &process.Program{Name: "pwd"}
+	require.NoError(t, p.Find())
+
+	dir := t.TempDir()
+
+	out, err := p.RunDir(testCtx(t), dir)
+	require.NoError(t, err)
+	// Resolve symlinks on both sides for portability (macOS uses /private/ prefix).
+	canonicalDir, err := filepath.EvalSymlinks(dir)
+	require.NoError(t, err)
+	canonicalOut, err := filepath.EvalSymlinks(out)
+	require.NoError(t, err)
+	assert.Equal(t, canonicalDir, canonicalOut)
+}
+
+func TestProgram_Run_FailingCommand(t *testing.T) {
+	p := &process.Program{Name: "false"}
+	require.NoError(t, p.Find())
+
+	_, err := p.Run(testCtx(t))
+	require.Error(t, err)
+}