feat: clickable Plan link + /gpr loop autonomous mode

[AdityaVG13]

Summary

Two small UX upgrades surfaced from real usage of the gpr loop:

• Clickable PRD link after grill. Once gpr-grill writes .gpr/Plan.json, it now unconditionally runs gpr render and prints the absolute .gpr/Plan.html path on its own line as file:///.... Modern terminals auto-linkify bare file:// URLs, so the user can open the rendered PRD in one click instead of copy-pasting gpr render --open.
• /gpr loop — hands-off mode. New routing in install/commands/gpr.md: /gpr loop, /gpr run, /gpr auto (and trailing flags) hand off to the existing gpr run CLI driver. The driver is a single deterministic binary that picks the next intent, spawns the build agent, parses the signal, runs the audit, persists, and repeats — identical for Claude/Codex/OpenCode/Gemini. The user's observation that "some AIs' loop function is not great" is exactly why this delegates to the CLI rather than asking Claude to self-loop.

Single-iteration (/gpr, /gpr next, /gpr ratchet) stays the default. Only the explicit loop/run/auto subform delegates.

Commits

1. feat(grill): print clickable file:// link to rendered Plan
2. feat(skill,cmd): route /gpr loop to autonomous gpr run CLI driver

Files

• install/commands/gpr.md — decision tree expanded with loop branch
• install/skill/SKILL.md — new "Autonomous loop — /gpr loop" section; routing block at top
• install/skill/gpr-grill/SKILL.md — final-write step now renders + prints clickable link

Test plan

• bash install/install.sh copies all three updated files to ~/.claude/{skills,commands}/
• In a project with .gpr/Plan.json: /gpr loop walks the new branch — preflight, budget surface, explicit yes, then gpr run --agent claude
• In a project without .gpr/Plan.json: /gpr-grill runs the nine beats, writes Plan.json, prints Plan ready: file:///.../Plan.html on its own line (clickable in iTerm2/Warp/VS Code)
• /gpr (no args) still runs exactly one iteration and yields
• Frontmatter parses on all three files (verified locally)