fix: /plan empty-target fast path — stop burning ~5k tokens in empty repos

[time-attack]

Problem

Running /plan in a totally empty repository burned ~5k tokens of reference and specialist-module reading before the agent acknowledged there was nothing on disk. The dispatch protocol mandates reading three reference files plus the full active module before any output, so the empty-tree discovery came last instead of first.

Fix

Dispatcher-level (scripts/gstack2/generate-skill-tree.ts, same gap/supplemental pattern as the Scale header): a plan-only "Empty-target fast path" section. One cheap ls -A probe runs before any reference or module read. An empty tree (or only VCS/tooling metadata) short-circuits to an immediate greenfield header, states the repo is empty in the first sentence, skips decision-store recall and every repo-reading phase, and asks the single what-do-you-want-to-build question when the prompt carries no idea. Codebase vector fixes at greenfield; scale comes from the prompt alone.

No specialist overlay and no EXPECTED_PARITY_CHECKS bump — the fast path runs before modules are read, so it lives in the dispatcher, not the legacy bodies.

Evidence

• bun run test:gstack2 passes (all 26 files, parity at 5027 checks)
• bun test passes (exit 0)
• Regen committed separately per bisection style; only skills/plan/SKILL.md (+4 lines) and the parity transcript hash changed

🤖 Generated with Claude Code

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Summary by cubic

Add an empty-target fast path for /plan that probes the repo with a single ls -A before any reads. Empty repos now short-circuit to a greenfield plan, saving ~5k tokens and time.

• Bug Fixes
  • Dispatcher pre-probes; if empty or only VCS/tooling files, skip decision-store recall, repo scans, and specialist phases.
  • Emit immediate header with Target: <path> (empty repository — greenfield); if no idea in the prompt, ask what to build.
  • Updated scripts/gstack2/generate-skill-tree.ts and skills/plan/SKILL.md; tests pass.

^{Written for commit 49b8687. Summary will update on new commits.}

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