Add --issue-pattern flag for custom subject identifier extraction

[RomainCscn]

Adds --issue-pattern=<regex> to sync: a user-supplied regex whose capture group 1 extracts issue identifiers from commit subjects, for conventions the built-in start-anchored patterns can't see — feat(routing)[ENG-123]: …, feat(api): ENG-621 …, suffixed … DEV-123 DEV-124.

• Applied to the subject only, case-insensitively, matched globally; additive to branch-name / magic-word / built-in subject detection and deduplicated through the same path, so behavior without the flag is unchanged.
• Captures go through matchAllIdentifiers, reusing shape validation, uppercasing, and leading-zero rejection; iteration uses matchAll, so zero-width-capable patterns can't loop.
• Validated at parse time: the regex must compile and contain a capture group. A scan where the pattern matches subjects but group 1 never yields a valid identifier warns instead of failing silently.
• Recorded in debugSink.issuePattern — requires linear/linear-app#83794 deployed first, since ReleaseDebugSinkInput is a closed input type.

Supersedes #117 (thanks @MatthewCline-git — same flag shape; this adds the zero-width guard, capture-group validation, the misplaced-group warning, scan-level tests, and docs).

Closes #106. Fixes #106

[linear-code]

FEA-12200