obs: evolve the size-report PR comment into a quiet quality-delta comment + metric history on main

[thymikee]

Part of #1412 (Track C). Depends on the repo-health snapshot issue.

Context

scripts/size-report.mjs already does the right thing: one marker-tracked sticky comment, updated in place. Evolve it into the single quality-delta surface instead of adding new bots. Hard requirement from the maintainer: do not drown people in information.

Deliverables

• Main-branch history: on every push to main, append the repo-health snapshot to a JSONL store (artifact chain, dedicated branch, or gh-pages — pick one and justify briefly in the PR).
• PR comment (evolved size-report): shows ONLY rows whose delta vs the main baseline crosses a per-metric threshold (size bytes, changed-line coverage, ratchet values, new fallow suppressions, slow-test budget breaches, redundant-edge count). When nothing crosses, the entire comment collapses to one line: "No notable quality deltas."
• Every shown row links the owning gate/fix command (e.g. a suppression row points at .fallowrc.json and the finding).
• Thresholds live in one config block; the comment never exceeds ~20 lines — overflow summarizes ("+3 more, see job summary").

Acceptance

• One sticky comment per PR, updated in place (reuse COMMENT_MARKER machinery).
• A PR with no notable deltas produces the one-line form.
• History survives force-push-free main flow and is queryable by commit sha.

Review amendments (2026-07-27)

• Specify fork-PR behavior explicitly: fork PRs have no secrets and read-only tokens — degrade to job-summary output (no comment) rather than failing.
• Serialize concurrent main-history writes: two merges racing must not lose an entry (workflow concurrency group, or compare-and-swap on the history ref with one retry). State the chosen mechanism in the PR.