feat(hooks): let driftcheck skip intentionally non-executable scripts

[ulises-c]

Summary

Adds ignore support to the driftcheck.sh Stop hook so repos with intentionally non-executable scripts (e.g. navecho_monorepo, where run scripts are chmod'd at their install destination) no longer block the end of every Claude Code session.

Closes #56

Design note — diverges from the issue's original spec

Issue #56 proposed a per-repo .driftcheckignore committed at each repo root. What this PR ships instead is a single global ignore file at ~/.claude/hooks/driftcheck-ignore, holding repo-relative glob patterns (one per line, # comments and blank lines allowed). This is the design that ended up in the working copy — it avoids committing a Claude-tooling dotfile into third-party/work repos.

Trade-off to be aware of when reviewing: because the file is global, its repo-relative patterns apply in every repo — e.g. workstation_install/*/settings.sh would also exempt an identically-shaped path in an unrelated project. If you'd rather have the per-repo semantics from the issue, say so and I'll swap the implementation.

Testing

• shellcheck --severity=warning clean (SC2053 explicitly disabled — unquoted RHS glob match is the point).
• Verified end-to-end in a scratch repo with an overridden $HOME:
  • no ignore file → violation flagged, exit 2
  • matching glob pattern → violation skipped, exit 0
  • non-matching pattern → still flagged, exit 2
  • ignored file plus a different violation → other violation still flagged, exit 2

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