feat(branding): add subtle Context Sheriff labels

[marcohefti]

This adds subtle, UI-only branding and reviewer-facing docs so the fork is immediately recognizable without changing Codex behavior.

Changes:

• TUI: session header shows Codex (Context Sheriff) with narrow-terminal fallbacks.
• CLI: --help / about text mentions the codex-context-sheriff fork.
• Docs: adds docs/context-sheriff.md as the canonical fork overview (thesis, features, non-goals).
• README: adds a small fork "front door" section + clarifies this fork is currently build-from-source only.

Constraints honored:

• No behavior changes (tools, compaction, approvals, auto-compaction).
• No new commands, prompts, or protocol events.