fix(cli): anchor test-report testPaths to the spec folder deterministically

[protonspy]

Summary

csdd spec test-report populated test-report.json's testPaths from the
workspace-relative paths of whichever JUnit/coverage artifacts were parsed or
auto-discovered (e.g. coverage.out, tests/junit.xml). That value was
non-deterministic — it drifted with --path and where the runner dropped files,
and could point outside the spec.

This change makes testPaths deterministically point at the spec's own folder:
exactly one entry specs/<feature>/ (forward-slash, trailing slash), set once
after the nothing-to-record guard, independent of artifact locations and of how
the metrics were gathered (--run discovery, --junit/--coverage, or explicit
counts). The now-orphaned appendUnique helper is removed; the stale TestPaths
doc comment in internal/session/reports.go is corrected.

Spec: specs/test-report-spec-paths/ (requirements → design → tasks all approved; csdd spec validate green).

Changes

• internal/cli/spec.go — set rep.TestPaths = []string{filepath.ToSlash(workspace.Relative(r, sdir)) + "/"}; drop the two appendUnique(rep.TestPaths, …Source) calls and the orphaned helper.
• internal/cli/test_report_test.go — assert the exact spec-folder value across --run, --junit/--coverage, and explicit-count origins; negative assertions that no artifact filename leaks.
• internal/session/reports.go — correct the now-false TestPaths doc comment (field/type unchanged).

Verification

- make check (gofmt + go vet + go test -race ./...)   ✅  (internal/cli 85.5%)
- csdd spec test-report (dogfood, rebuilt binary)      ✅  testPaths == ["specs/test-report-spec-paths/"], cov 81.2%
- code-reviewer sub-agent                              ✅  APPROVE, 0 Blockers

Risks

• Behavior change to a recorded field: any consumer that read testPaths expecting artifact file paths now gets the spec directory. The only in-repo consumer is the csdd web dashboard, which renders it as a string[] — unaffected (a one-element array is still valid). test-report.json is regenerated on the next run.

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