docs: ci/release tooling doc-accuracy batch

[dipto0321]

Summary

Four small CI/release tooling doc-accuracy fixes surfaced by issue
#69. Each is a one-line-to-small edit; the value is consistency
between the docs and the actual repo state.

1. CONTRIBUTING.md removed a bogus "CoGitto status check" and
   corrected the CI matrix count. The previous text said
   "10-check matrix" including "1 CoGitto status check" — no
   such workflow exists in .github/workflows/ (only ci.yml +
   release.yml). Worse, this contradicted CONTRIBUTING.md's own
   "Sign-off (future)" section (lines 141-146) which correctly
   notes CoGitto-style enforcement is "not required today; will
   be enabled by a .github/workflows/dco.yml check before
   v1.0.0." If branch protection were ever configured to require
   a check literally named "CoGitto," it would hang forever.
   Updated the matrix to 9 checks (5 builds + 1 lint + 3 OS
   tests).

2. make ci description in CONTRIBUTING.md was wrong. It
   said make ci runs tidy, vet, test, and
   golangci-lint run — omitting the fmt step and misstating
   the order. The Makefile target (line 126) is tidy fmt vet lint test. Since fmt rewrites files in place, the new
   description explicitly warns contributors to run it on a
   clean tree or be ready to commit the rewrites. CLAUDE.md's
   description was already correct and is unchanged.

3. "6 binaries" was wrong in three places. The actual platform
   matrix (linux/windows/darwin × amd64/arm64, minus the
   documented windows/arm64 exclusion in .goreleaser.yaml)
   produces 5 archives; checksums.txt is a 6th file but isn't a
   binary. Corrected in:

   • .goreleaser.yaml header comment — lists 5 archive names
     explicitly and notes windows/arm64 is intentionally excluded.
   • .github/workflows/release.yml step-3 comment — "all 5
     platform binaries" with a pointer at the windows/arm64
     exclusion in .goreleaser.yaml.
   • docs/release-checklist.md post-tag checklist — "All 5
     binaries attached + checksums.txt."

4. CI coverage filename reconciled with documented make test.
   .github/workflows/ci.yml was producing and uploading
   coverage.txt; make test (the documented local command)
   produces coverage.out. Renamed CI to coverage.out so a
   contributor running the documented make test sees the
   same filename CI consumes. Both filenames are gitignored so
   this is just naming consistency.

Linked issues

Closes #69

Type of change

• docs — documentation only
• feat — new feature (MINOR bump)
• fix — bug fix (PATCH bump)
• refactor — no behavior change
• perf — performance improvement
• test — tests only
• chore — maintenance / dependency bump
• ci — CI/CD only
• build — build system only
• Breaking change (MAJOR bump) — explain below

Checklist

• Title follows Conventional Commits (docs(scope): subject)
• I ran make ci locally and it passes
• I added or updated tests for the change (no test added —
  docs-only / CI-config-only change)
• I updated relevant docs (this PR is the doc update;
  CHANGELOG.md gets a ### Fixed entry under
  [Unreleased])
• No new linter warnings (make lint green)
• If breaking: I documented the migration path in the PR
  body and updated CHANGELOG.md

Files touched

• CONTRIBUTING.md — drops the bogus "CoGitto status check,"
  corrects matrix count to 9, corrects make ci description
  (adds fmt, fixes order, warns about in-place rewrites).
• .goreleaser.yaml — header comment lists 5 archive names
  with a note about the windows/arm64 exclusion.
• .github/workflows/release.yml — step-3 comment says "all 5
  platform binaries."
• docs/release-checklist.md — "All 5 binaries attached +
  checksums.txt."
• .github/workflows/ci.yml — renames coverage.txt →
  coverage.out so CI matches the documented make test
  output.
• CHANGELOG.md — ### Fixed entry under [Unreleased]
  documenting the doc refresh with docs: CI/release tooling doc-accuracy batch (CoGitto check doesn't exist, 6 vs 5 binaries, make ci description, coverage filename) #69 reference.

[cocogitto-bot]

✔️ ebedf63 - Conventional commits check succeeded.