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

[MatthewCline-git]

Summary

Closes #106.

The three built-in subject patterns in COMMON_SUBJECT_PATTERNS are all anchored to the start of the subject line, so teams that follow Conventional Commits — where the identifier appears after the type and scope — get no issue linked:

feat(routing)[ENG-123]: add stop reordering  ← not matched
fix[ENG-123]: handle empty payload           ← not matched

This adds a --issue-pattern flag that accepts a regex where group 1 captures the issue identifier. It's applied to the commit subject anywhere (not just the start), additive to the existing patterns:

linear-release sync --issue-pattern "\[([A-Z]+-[0-9]+)\]"

The pattern is validated as a legal regex at CLI entry. Invalid patterns produce a clear error:

Invalid --issue-pattern regex: Invalid regular expression: ...

Changes

• args.ts — new --issue-pattern flag, validated and returned as issuePattern: string | null
• extractors.ts — matchCustomSubjectPattern() applies the user regex globally to the subject, validates group 1 captures through the existing matchAllIdentifiers path; wired into extractLinearIssueIdentifiersForCommit via an optional second argument
• scan.ts — issuePattern added to ScanOptions, compiled once before the commit loop, threaded into the extractor
• index.ts — destructured from parsed args, passed to scanCommits, added to help text

[DanCorvesor]

Tried this branch against a real repo (GitLab CI + semantic-release, Conventional Commits) — --issue-pattern "([A-Za-z]{2,}-[0-9]+)" resolved every ticket the built-in leading-key patterns miss: mid-subject feat(api): asp-621 … and parenthesised (ASP-603) alike. Exactly the gap from #106. 🙌

One thing to fix before it's usable for live syncs: the actual releaseSyncByAccessKey mutation fails with

Field "issuePattern" is not defined by type "ReleaseDebugSinkInput"

because issuePattern is added to DebugSink, which is sent in the mutation input (--dry-run hides it since it skips the mutation). Opened a small PR against this branch that strips it from the payload while keeping it in the local sink for --verbose: MatthewCline-git#1. With that applied, live sync created real releases with all issues attached.

[RomainCscn]

Closing in favor of #124