Lint baseline timing and lint-gate summary (split from #443) #496

Split out of #443 (2026-10-20). PR #494 fixes #443's exit codes (lint-only = 4, UNVERIFIED = 5). These asks from #443 remain open; the review of #494 confirmed that no skill or script covers them yet.

1. When to take the first lint baseline. Take it with the user right after the first scaffold step, so later rises measure the build rather than the template. Today lint-that-actually-runs.md:140 and lint-gate.sh:51 say only that a first baseline needs no reason.
2. Summarise lint-gate output when most runs rise. In the field run that prompted [field 2026-10-17] exec.sh exits 1 on LINT ROSE with a clean mxbuild (frozen baseline) #443, about 90% of runs reported a rise. Print a one-line summary per rule, with the detail in .claude/loop/lint-last.json, so the signal isn't lost in the noise.
3. (Optional) An opt-in "hard lint gate" for projects that want a lint rise to stop the step (the old exit 1), e.g. Lint gate: hard in PROJECT.md.

Related: #348 (baselines can be loosened by the agent; draft #375).

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