move @docs to docs mode

[kalil0321]

Follow-up tracked from PR #63 (tag-system removal). The @docs tag was removed at the user-facing surface, but the underlying output_mode="docs" codepath is intentionally left dormant so a dedicated docs subcommand can be wired without re-implementing the engine.

What's already in place (dormant)

Location	What it does
engineer.run_reverse_engineering(..., output_mode="docs")	SDK dispatch already accepts output_mode
base_engineer._get_existing_client_guidance	Switches existing_label/existing_artifact between "scripts"/"docs" automatically
cli.run_engineer(..., output_mode="client")	Internal helper accepts output_mode, but no caller passes "docs" today
prompts/engineer/user.md	Template variable tag_mode_label flips to "Documentation" when docs mode is active

What the follow-up PR needs

1. New click subcommand docs:
   • Args/flags mirroring engineer: positional run_id (optional — defaults to latest run), -p/--prompt (override), -m/--model, -o/--output-dir
   • When called: run_engineer(run_id, output_mode="docs", ...)
   • Latest-run resolution: session_manager.get_history(limit=1)[0]["run_id"] when no positional given (mirrors what the old @docs standalone tag did)
2. HAR-existence guard: the old @docs path validated that recording.har exists for the target run before running. Replicate that check in the new subcommand to keep the same error UX.
3. Output directory: docs go under get_docs_dir(run_id, output_dir) (already exists in utils.py). Make sure the new subcommand surfaces the resulting path on success.
4. README: add a Docs Mode section under Usage Modes between Engineer Mode and Agent Mode.
5. CHANGELOG (Unreleased): document the new docs subcommand and reference this issue.
6. Tests: a tests/test_cli_docs_command.py mirroring the structure of test_cli_engineer_command.py — covers docs (no args → latest run), docs <run_id>, docs <run_id> -p "...", missing-HAR error.

Out of scope here

• Reintroducing the @docs REPL tag — the principle from cleanup: remove tag system, expose equivalents as CLI flags #63 is that capabilities live as first-class subcommands/flags, not inline tags.
• Changing the prompt template structure (already cleaned up in cleanup: remove tag system, expose equivalents as CLI flags #63).

References

• PR cleanup: remove tag system, expose equivalents as CLI flags #63 — tag system removal (parent cleanup, where the @docs tag was retired)
• Issue agent friendliness #62 — agent-friendliness backlog (related: docs --json surface for agents would be nice eventually)