feat(stats): add surf stats adoption metrics (#13)

[Connorrmcd6]

Summary

The proposal's falsifiable success/kill criteria (§9.2) hinge on two metrics, and the rubber-stamp rate is the one that distinguishes a working gate from one being routed around. Nothing computed them. This adds surf stats.

surf stats                                       # all history
surf stats --since 2026-01-01 --format json

Metrics

• Rubber-stamp rate — of re-stamp events (a commit changed a claim's stored hash: to a new value), the share where the claim's prose was left untouched. A rising rate is a kill signal.
• In-place update rate — of claim-touch events (a commit changed a file a claim anchors), the share where the claim's hash was updated in the same commit.

Implementation

• Walks each non-merge commit in the --since/--until window (one commit ≈ one PR), parsing the hub set as it existed at the commit and its parent via git ls-tree + git show, then comparing stored hashes and prose across the pair.
• New best-effort git helpers in git.rs: log_commits, commit_files, list_files_at (same None-on-failure pattern as the existing ones).
• JSON emits a versioned {version, since, until, commits, rubber_stamp, in_place} envelope; each rate is {n, d, rate} with rate: null when d == 0.
• Unlike the advisory git in check, stats is a history report: it errors (non-zero) when git history is unavailable rather than printing a misleading zero.

Heuristics (surfaced, not hidden)

Documented in docs/guides/stats.md: one commit = one PR (merges excluded); claim identity is its at: site(s); the in-place denominator counts any anchored-file edit (including hash-neutral ones), so the reported in-place rate is a floor.

Verification

• Unit tests build temp git repos exercising a rubber-stamp commit, a genuine prose+hash update, an anchored-file touch with no re-seal, the not-a-repo error path, and the versioned JSON shape; full suite green.
• Live on this repo: surf stats returns plausible numbers (55% rubber-stamp, 47% in-place over all history).
• Dogfood: added hubs/cli-stats.md + anchored the new git helpers in hubs/cli-git.md; surf lint/surf check stay green (CLI command-list hub updated for stats).

Closes #13.

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