docs(spec-loop): align reviewer-routing coverage claim

[Whxuan0701]

Summary

• Correct the non-ASF reviewer-routing coverage statement in tools/spec-loop/specs/reviewer-routing.md.
• Describe the behavior actually asserted by the fixtures: area-match routing selects a primary reviewer, while no-area-match returns NO ELIGIBLE REVIEWER without proposing a fallback.
• Leave the skill implementation and all eval fixtures unchanged.

Validation

• uv run --project tools/spec-validator spec-validate tools/spec-loop/specs/
• uv run --project tools/skill-evals skill-eval tools/skill-evals/evals/non-asf-profile-smoke/step-reviewer-routing/fixtures/
• uv run prek run --all-files
• git diff --check

All checks passed, including spec validation, the focused reviewer-routing eval, lychee, markdownlint, skill-and-tool validation, and the repository test suite.

Known limitations

• Documentation-only change; it does not add or change load-aware fallback behavior.
• The upstream Issue could not be assigned to this contributor because the account has fork/pull permissions but not issue-assignment permissions.

Generative AI disclosure

This PR was prepared with AI assistance and reviewed against the repository source-of-truth fixture and validation commands. The commit carries the required Generated-by: Codex (GPT-5) trailer.

Closes #999