test(coverage): enforce an 80% c8 floor and cover the CLI surface (#85)

[robotlearning123]

What

Closes #85. test:coverage had no threshold, so it always exited 0. src/commands/* and src/index.ts never appeared in the report because no test loaded them.

• New .c8rc.json:
  • all: true, so unloaded src files show as 0% instead of disappearing.
  • check-coverage with lines: 80 and functions: 80.
  • It excludes only files with no runtime logic: src/types.ts (interfaces), src/lib.ts, the pure re-export barrels src/engine/index.ts and src/utils/index.ts, and *.d.ts.
  • src/index.ts (CLI entry) and src/templates/index.ts (template loader) stay inside the gate.
• New test/commands.test.ts:
  • In-process tests for checkCommand and initCommand, with console and process.exit stubbed. They cover human and --json output, --strict exit 1, the error branch, --dry-run, --check, --force, template substitution, an invalid package.json, and a write failure.
  • tsx-spawned tests for src/index.ts routing.
  • Config tests pin that src/index.ts and src/templates/index.ts are never excluded.
• package.json is not modified. The threshold lives in .c8rc.json, so it does not collide with deps: bump c8 from 10.1.3 to 11.0.0 #55 (c8 10->11) or fix(test): include test/e2e in test globs and build before tests #90 (e2e test globs).

Evidence (this branch)

Run	Result
main before	npm run test:coverage exits 0, with no src/commands / src/index.ts rows
config only, no new tests	ERROR: Coverage for lines (65.74%) does not meet global threshold (80%), exit 1
after	77/77 tests pass; All files 84.04% lines / 84.12% funcs; exit 0; npm run check green
regression probe	removing test/commands.test.ts gives exit 1

Sequencing

• Branched from origin/main e704b49. This PR does not depend on fix(test): include test/e2e in test globs and build before tests #90 or deps: bump c8 from 10.1.3 to 11.0.0 #55.
• deps: bump c8 from 10.1.3 to 11.0.0 #55: .c8rc.json uses standard c8 options. They were run with c8 10 only; after deps: bump c8 from 10.1.3 to 11.0.0 #55 lands, re-run npm run test:coverage.
• fix(test): include test/e2e in test globs and build before tests #90: the e2e tests are still outside npm test. When fix(test): include test/e2e in test globs and build before tests #90 lands, coverage can only rise, so re-measure the floor then.
• CI runs npm test, not test:coverage, so the floor is not yet enforced in CI. That is a CI change left for an owner decision.

Review

• An independent reviewer found a major issue: the original exclude glob src/**/index.ts also matched src/index.ts. It was fixed in e608f76, and the delta re-review said SHIP.
• A follow-up minor finding said src/*/index.ts still hid src/templates/index.ts. That was fixed in 72f4634, with a test that fails first.
• Known minor: the config test uses path.matchesGlob, which needs Node >= 20.17 / 22.5. CI's 20.x / 22.x resolve to recent releases.