docs(site): document safe-outputs key validation errors

[github-actions]

Summary

• Users now know that typos and old/renamed keys under safe-outputs: fail compilation immediately with a hint listing similar known tools — preventing the previous silent-drop behavior.
• A caution callout in the Safe Outputs reference makes the compile-time key contract discoverable at the point of configuration.

Changes

• site/src/content/docs/troubleshooting/common-issues.mdx:
  • Add "safe-outputs contains unrecognised tool name(s)" entry with the exact error format, a before/after YAML example (create-pr → create-pull-request), and a link to the full tool list.
  • Add "safe-outputs contains tool name(s) with invalid characters" entry showing the character constraint and a corrected example.
• site/src/content/docs/reference/safe-outputs.mdx:
  • Add :::caution[Compile-time key validation] callout in the intro section stating that all keys must be exact registered names and that the compiler rejects unknowns with a similar-tools hint.

Accuracy checks

• Error messages transcribed directly from src/compile/common.rs (validate_safe_outputs_keys function — added in PR feat(safeoutputs): add daily smoke suite and reject unknown safe-output keys #563).
• create-pr → create-pull-request rename confirmed in src/safeoutputs/mod.rs (ALL_KNOWN_SAFE_OUTPUTS contains create-pull-request, not create-pr).
• Character constraint (ASCII letters, digits, hyphens) confirmed against validate::is_safe_tool_name call in the same function.

Validation

• cd site && npm ci && npm run build — 29 pages built, all internal links valid.

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