feat: volume/cadence context and time-bucketed trends (#4)

[gkanitz]

Closes #4

Summary

Adds the volume/cadence context section: active days, contribution counts, and a monthly time-bucketed trend series scoped to the half-open coverage window — presented strictly as context, subordinate to collaboration metrics, with no composite score anywhere.

• metrics/cadence.go — cadence concern registered into the metrics registry; flattens the subject's own timestamped events (PRs authored, reviews given, review comments written; activity received is excluded) and computes active UTC days + contribution counts.
• metrics/buckets.go — generic monthly bucketing over timestamped events: UTC calendar-month buckets clamped to the window (partial first/last buckets), half-open [start, end) semantics. Generic over the event series tag, so collaboration trend series can be wired after Full GitHub collaboration metrics (reviews, comments, merge timing, rework) #3 lands without touching Full GitHub collaboration metrics (reviews, comments, merge timing, rework) #3's files.
• report — Cadence sub-struct gains contributions and trend (TrendBucket{start, end, counts}); additive only.
• render — 40-cadence.tmpl renders cadence as a muted section.context after collaboration, with a small trend table; numbers never get the headline stat treatment. Additive CSS in layout.tmpl, additive total helper in the funcs map.
• cmd/coderepute/main.go — pre-approved one-line wiring: passes &result.Cadence instead of nil.

Acceptance criteria

• Report JSON includes active days, contribution counts, and time-bucketed trend series scoped to the coverage window (TestCadenceJSONShape, TestRunEndToEnd)
• Rendered HTML presents volume/cadence visually subordinate to collaboration metrics (TestHTMLCadenceIsSubordinateContext: ordering after collaboration, context styling, no headline stat classes)
• Bucket-boundary edge cases covered by table-driven metrics tests (TestComputeCadenceTrendBucketBoundaries: UTC normalization of offset timestamps, partial first/last buckets, inclusive bucket starts / exclusive window until, sub-month windows)
• No composite or aggregate "score" in schema or rendering (asserted in both the e2e JSON check and the render test)

Parallel-work contract

• ActivitySet untouched; collaboration metric files, coverage/verification sub-structs, repo-enumeration/token-auth, CI YAML untouched.
• Shared files touched additively only: report/report.go (Cadence fields + TrendBucket), render/render.go (one FuncMap entry), render/templates/layout.tmpl (CSS lines appended), cmd/coderepute/main.go (the pre-approved one-liner), plus additive test cases in report_test.go, render_test.go, main_test.go.

Test output

$ go test -count=1 ./...
ok  	github.com/grkanitz/coderepute/cmd/coderepute	1.107s
ok  	github.com/grkanitz/coderepute/metrics	0.205s
?   	github.com/grkanitz/coderepute/provider	[no test files]
ok  	github.com/grkanitz/coderepute/provider/github	0.498s
ok  	github.com/grkanitz/coderepute/render	0.655s
ok  	github.com/grkanitz/coderepute/report	0.801s

$ go vet ./...
(clean)

🤖 Generated with Claude Code

[gkanitz]

Review round 1 — approved

Verified independently in a clean worktree: go build ./..., go vet ./..., go test -count=1 ./... all pass.

• JSON criterion: cadence carries active_days, contributions, and a trend series of half-open [start, end) buckets clamped to the coverage window (report/report.go, metrics/cadence.go, metrics/buckets.go); asserted by TestCadenceJSONShape and the e2e test.
• Subordinate rendering: 40-cadence.tmpl renders after 30-collaboration.tmpl inside section.context (muted, small type); only collaboration uses the stat/n headline classes, and TestHTMLCadenceIsSubordinateContext locks this in.
• Bucket edge cases: table-driven tests cover UTC normalization of offset timestamps (recomputed the -05:00 boundary case by hand — correct), partial first/last buckets, inclusive bucket starts vs. exclusive window until, and sub-month windows.
• No composite score: the word appears only in disclaimers; e2e test rejects any "score" JSON key.
• Contract: diff touches only owned files plus additive changes to report/report.go (Cadence sub-struct, owned), render/render.go (one FuncMap entry), layout.tmpl (appended CSS), and the single pre-approved main.go line. ActivitySet, collaboration metrics, coverage/verification, provider, and CI YAML are untouched.

Formal approval is not possible on a same-account PR; merging by squash.

🤖 Generated with Claude Code