Improve one-liner → full overlay UX + vendored engine docs

[FineComputer14451]

Summary

Closes the gap between pure one-liner installs and the full Coding Team / skills / menu experience, and documents reproducible offline installs via a vendored termux-distro engine.

Included in this PR

File	Change
lib/actions.sh	Post-install banner detects missing bin/+skills/+agents/ and tells the user to clone + grokhunter skills install
README.md	Recommended full stack command is first; one-liner marked “core only”; new Reproducible / offline section; vendor/ in architecture tree
docs/ARCHITECTURE.md	Engine resolution priority includes vendor/; SHA pin docs
vendor/README.md	New — how to vendor termux-distro.sh + optional integrity pin
docs/UX-VENDOR-PATCH-NOTES.md	Notes for remaining install.sh / doctor / grok.sh code patches

Follow-up (code, not yet in this PR)

Full diffs were prepared in review for:

1. install.sh → resolve_distro_engine() preferring vendor/termux-distro.sh + optional GROKHUNTER_DISTRO_ENGINE_SHA256
2. lib/grok.sh → clearer incomplete-overlay warnings in install_cli_bins
3. bin/grokhunter-doctor → new Overlay completeness section

Those can land in a second PR or be applied locally from the conversation diffs.

Test plan

• bash install.sh --overlay-only --with-completions from a clone → no “full overlay missing” banner
• Pure one-liner path still works; post-install shows the clone + skills install nudge
• README “Recommended install” is the first command a new visitor sees
• mkdir -p vendor && curl … -o vendor/termux-distro.sh documented and ready for engine-pin follow-up

Credits

Upstream engine remains jorexdeveloper/termux-distro (GPL-3.0). See CREDITS.md.