feat: deliver pinned files to coding turns and inspect admission

[AetherAI3]

Closes #324.

What changed

• Read explicit pins once at turn admission from the execution checkout. Project-relative binding prevents an original-checkout file from leaking into a worktree run.
• Admit complete UTF-8 files only under 64 KiB per file, 32 pins, and a 512 KiB aggregate composed-context budget with rules and skills. Report every omission and the admitted digest, byte range, size, route, and workspace binding.
• Add /context metadata inspection, /context next <task> unsent preview, and explicit /context content <path> local content preview. Keep file bodies out of registry snapshots and generic metadata.
• Route the single frozen file block through the existing local Ollama and hosted dev-session brief. Mark server-executed cloud chat as unsupported because the local host cannot prove its selected-file admission.

Evidence

• npm run docs:check passed.
• npm run build passed.
• 88 scoped tests passed across selected-context, skill host loop/policy, command manifest, slash registry, and registry scope suites.
• Fixtures cover worktree binding, drift after admission, local/hosted single inclusion, Unicode and fence safety, missing/binary/invalid/oversized files, aggregate overflow, symlink escapes, /drop, and metadata-only inspection.