[FEAT] Add npx install, resolve-conflicts skill, and session-commit skill

[Olshansk]

tl;dr Adds npx install path, resolve-conflicts skill, and session-commit skill to make gstack easier to adopt and more useful day-to-day.

Summary

• Instant install via npx: Users can now run npx skills add garrytan/gstack to get all skills without building the browser binary or downloading Chromium
• Conflict resolution skill: New /resolve-conflicts skill with 3-tier classification — auto-resolves trivial conflicts, explains non-trivial ones, escalates ambiguous cases
• Session commit skill: New /session-commit skill captures learnings from coding sessions and persists them to AGENTS.md for future sessions

Feature Diff

Component	Before	After
Install path	git clone + ./setup only	npx skills add (instant) + full install option
skills.sh listing	N/A	Linked at skills.sh/garrytan/gstack
/resolve-conflicts	N/A	3-tier merge conflict resolver with auto-resolve, explain, and escalate modes
/session-commit	N/A	Session learning capture → AGENTS.md updates with preflight checks

Details

Technical Details

npx Install Path

• Added "Skills only" section to README above the existing full install instructions
• Links to skills.sh catalog page for discoverability
• Skills that don't need the browse binary (/review, /ship, /retro, /plan-*) work immediately via npx

resolve-conflicts Skill

• resolve-conflicts/SKILL.md: 3-tier conflict classification (trivial → non-trivial → ambiguous)
• Auto-resolves lock files, generated files, and whitespace-only conflicts
• Explains reasoning for non-trivial resolutions before applying
• Escalates via AskUserQuestion when both sides have meaningful changes

session-commit Skill

• session-commit/SKILL.md: Analyzes session diffs and conversation to extract patterns
• session-commit/scripts/preflight.sh: Validates AGENTS.md exists and is well-formed
• session-commit/references/change-proposal-format.md: Structured format for proposed AGENTS.md changes