signals: add direct unit-test coverage for buildReviewRiskExplanation's 5-way branch logic

[JSONbored]

⚠️ Definition of Done: this issue must be completed in full, in a single PR. Do not split this
work across multiple PRs, and do not defer any Deliverable below to a follow-up issue. A PR that
satisfies only some of the Deliverables, stubs a required test, or leaves a checkbox
partially-done does NOT resolve this issue and will be closed.

Context

buildReviewRiskExplanation in src/signals/review-risk.ts (lines ~29-56) is a pure function
implementing a 5-way recommendation decision: likely_duplicate → maintainer_lane →
needs_author → review → watch. It is advisory only — it does not gate, dispatch, or affect
merge/close decisions, it only produces an explanation string surfaced to callers.

Despite the real branching logic, it has no direct unit test exercising each branch:
test/unit/routes-review-risk.test.ts calls buildReviewRiskExplanation only to compute its own
"expected" value, then asserts the REST/MCP routes match that self-same computed value — a
parity check between two callers, not a correctness check of the function's branch logic itself.
test/unit/support/mcp-cli-harness.ts only exercises a fixture-mocked HTTP server, never the real
function. No other test file in test/unit/ imports or calls buildReviewRiskExplanation
directly.

A clean precedent for the kind of direct, small unit-test file this needs already exists in the
same directory family: test/unit/contributor-readiness-band.test.ts directly unit-tests a
similarly-sized pure function from src/signals/.

Requirements

• Add direct unit test coverage for buildReviewRiskExplanation that exercises each of the 5
  possible recommendation outcomes (likely_duplicate, maintainer_lane, needs_author,
  review, watch) by constructing the minimal PreflightResult/RoleContext-shaped input
  needed to deterministically produce each branch — not by mocking or stubbing the function
  itself.
• Do not modify buildReviewRiskExplanation's implementation — this issue is coverage-only. If
  writing the tests reveals an actual bug in the branch logic, note it in the PR description but
  still land the coverage; do not silently "fix" behavior as a side effect without it being an
  explicit, separately-called-out change in the PR body.
• Tests must live in a new or existing test/unit/ file that imports the real function from
  src/signals/review-risk.ts directly (mirror test/unit/contributor-readiness-band.test.ts's
  structure) — not through the REST/MCP route layer.

Deliverables

• A test file (new test/unit/review-risk.test.ts, or an addition to an existing
  src/signals-scoped test file if one already directly tests sibling functions in
  review-risk.ts) with one test case per recommendation branch (5 total), each asserting
  both the recommendation value and the explanation text/shape returned for that branch.
• Each test constructs its own minimal fixture input rather than reusing
  routes-review-risk.test.ts's fixtures wholesale, so the test is a real correctness check
  independent of that file's parity assertions.

Both deliverables are required in this single PR.

Test Coverage Requirements

This repo enforces 99%+ Codecov patch coverage, branch-counted, on every changed line/branch in
src/**. Since review-risk.ts itself is not being modified, this issue's own patch-coverage
surface is the new test file — it must actually execute (not stub) all 5 branches of
buildReviewRiskExplanation.

Expected Outcome

buildReviewRiskExplanation's 5-way branch logic has direct, real unit-test coverage independent
of the existing route-parity checks, so a future regression in the branch logic itself (not just a
route/function mismatch) would be caught.

Links & Resources

• src/signals/review-risk.ts (function under test, ~lines 29-56)
• test/unit/contributor-readiness-band.test.ts (structural precedent to mirror)
• test/unit/routes-review-risk.test.ts (existing parity-only test — do not duplicate its
  approach, this issue needs direct correctness coverage)

[linear-code]

JSO-6442