fix(rfc-025): add os to Principle 9 quality gate

os bypasses Fs/Env primitives. Core provides:
- c.Fs().Write/Read/List/EnsureDir/TempDir/DeleteAll
- core.Env() for environment variables
- core.DirFS() for fs.FS from directory

Validated: core/go tests have zero os imports.

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

docs/specs/RFC-025-AGENT-EXPERIENCE.md

@@ -402,6 +402,7 @@ Core primitives become mechanical code review rules. An agent reviewing a diff c
 | `os/exec` | Bypasses Process primitive | `c.Process().Run()` |
 | `unsafe` | Bypasses Fs sandbox | `Fs.NewUnrestricted()` |
 | `encoding/json` | Bypasses Core serialisation | `core.JSONMarshal()` / `core.JSONUnmarshal()` |
+| `os` | Bypasses Fs/Env primitives | `c.Fs()`, `core.Env()`, `core.DirFS()` |
 | `path/filepath` | Bypasses path security boundary | `core.Path()` / `core.JoinPath()` |
 | `fmt.Errorf` | Bypasses error primitive | `core.E()` |
 | `errors` | Bypasses error primitive | `core.NewError()` / `core.Is()` / `core.As()` |