docs(action): Add action README and review guide for review mode

[mhiro2]

Summary

• Add action/README.md as the action's reference (inputs, outputs, exit codes for both diff and review modes, plus the two-pass review implementation note).
• Rework docs/github-actions.md into a workflow-wiring guide that links to the new reference and to the sticky-comment sample, with no duplicated input/output tables.

Changes

• 40e8faf : docs(action): add action README and review guide for review mode
  • Introduce action/README.md covering quick start (diff and review), full input reference (mode-aware), output reference, exit-code semantics for both modes, two-pass review explanation, locally built binary recipe, and troubleshooting (install rate limits, dialect inference, format compatibility, sticky comment cleanup).
  • Rewrite docs/github-actions.md as a guide: dual-mode quick start, schema materialization steps shared by both modes, "Reviewing migrations on pull requests" walkthrough that distills the patterns embedded in action/examples/relune-review.yaml, expanded fork PR section with skip / workflow_run / pull_request_target options, and a unified sample-workflow table covering diff and review.
  • Note that @v0 does not resolve until the first tagged release is cut and direct readers to @main or a commit SHA in the meantime.

[github-actions]

Code Metrics Report

	main (2e6e20c)	#95 (75fa94a)	+/-
Coverage	94.6%	94.6%	0.0%
Test Execution Time	1m52s	1m32s	-20s

Details

  |                     | main (2e6e20c) | #95 (75fa94a) | +/-  |
  |---------------------|----------------|---------------|------|
  | Coverage            |          94.6% |         94.6% | 0.0% |
  |   Files             |             81 |            81 |    0 |
  |   Lines             |          36827 |         36827 |    0 |
  |   Covered           |          34864 |         34864 |    0 |
+ | Test Execution Time |          1m52s |         1m32s | -20s |

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Reported by octocov