fix(skill-nudge): log swallowed errors in debug mode

[yuhaouno]

Adds debug-only stderr diagnostics for failed profile lookups and unreadable managed skill targets, while preserving default output and exit status. Wires the parsed global --debug flag into the pre-action nudge. This is the narrow skill-nudge subtask of #186. Verified with full tests, lint, typecheck, and format:check. Closes #285.

[github-actions]

✅ This PR is linked to an issue assigned to @yuhaouno — thanks! The needs-issue label has been removed.

[coderabbitai]

Walkthrough

Changes

Skill nudge debug diagnostics

Layer / File(s)	Summary
Managed target read diagnostics src/lib/skill-nudge.ts	The skill presence probe reports managed target read failures through an optional, failure-isolated callback.
Debug propagation and validation src/lib/skill-nudge.ts, src/index.ts, src/lib/skill-nudge.test.ts	The CLI forwards --debug, nudge handling emits debug messages for swallowed errors, and tests cover debug-enabled and disabled output.

Estimated code review effort: 2 (Simple) | ~10 minutes

Sequence Diagram(s)

sequenceDiagram
  participant CLI as preAction hook
  participant Nudge as maybeEmitSkillNudge
  participant Probe as isVerifySkillInstalled
  participant Output as write sink
  CLI->>Nudge: pass debug option
  Nudge->>Probe: check skill presence with onReadError
  Probe->>Nudge: report managed target read failure
  Nudge->>Output: emit debug diagnostic
  Nudge->>Output: emit existing skill warning

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Suggested reviewers: ruili-testsprite, zeshi-du

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: debug-mode logging for swallowed skill-nudge errors.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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