fix(plan): only show progress from current step

[MathurAditya724]

getProgressMessage() walked backwards through all autofix steps and returned the first progress message found from any step. During solution polling after RCA completes, this surfaced stale RCA messages (e.g. "Looking at src/mdx.ts...") because the completed RCA step still had progress entries while the new solution step had none yet.

Now only reads progress from the last (current) step and falls back to a status-based message when it has no progress.

Testing

bun test test/lib/formatters/seer.test.ts — 40 tests pass, including two new tests for the exact scenario from the issue.

Fixes #950

[github-actions]

PR Preview Action v1.8.1
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[github-actions]

Codecov Results 📊

✅ 6858 passed | Total: 6858 | Pass Rate: 100% | Execution Time: 0ms

📊 Comparison with Base Branch

Metric	Change
Total Tests	📈 +2
Passed Tests	📈 +2
Failed Tests	—
Skipped Tests	—

All tests are passing successfully.

✅ Patch coverage is 80.00%. Project has 14030 uncovered lines.
❌ Project coverage is 77%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
src/lib/formatters/seer.ts	80.00%	⚠️ 1 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
- Coverage    77.04%    77.00%    -0.04%
==========================================
  Files          319       319         —
  Lines        60950     60988       +38
  Branches         0         0         —
==========================================
+ Hits         46956     46958        +2
- Misses       13994     14030       +36
- Partials         0         0         —

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Generated by Codecov Action

[MathurAditya724]

All 18 CI checks passed (unit tests, e2e, lint, typecheck, build, CodeQL, semgrep). Self-review confirmed the fix is correctly scoped — only getProgressMessage() changed, no regression for the explain flow since single-step RCA still works via steps.at(-1). Two new tests cover the exact bug scenario from #950. Marking ready for review.