feat: add --issue-pattern to extract issue IDs from custom subject formats

[olsavmic]

Closes #106.

What & why

Issue identifiers are only extracted from a commit subject when the identifier leads the subject — the three built-in COMMON_SUBJECT_PATTERNS are start-anchored ([ENG-123] …, (ENG-123) …, ENG-123: …). Teams using Conventional Commits put the identifier after the type/scope, so it is silently dropped:

feat(routing)[ENG-123]: add stop reordering
fix[ENG-123]: handle empty payload

--include-subjects can't recover these — it's a filter (which commits to scan), not an extractor (where the identifier is).

This adds --issue-pattern=<regex> for sync: a user-supplied regex whose group 1 is the team key and group 2 is the issue number.

linear-release sync --issue-pattern="\w+(?:\([^)]*\))?!?\[(\w+)-(\d+)\]"

Behavior

• Matched against the commit subject (first line), like --include-subjects and the built-in subject patterns.
• Scanned globally → a subject can yield multiple identifiers.
• Case-insensitive; team key upper-cased, leading zeros stripped (eng-0045 → ENG-45); leading-zero numbers rejected — same normalization as parseMatch.
• Additive: merged with branch-name / magic-word / built-in-subject detection and de-duplicated. Default behavior is unchanged when the flag is absent.
• Validated at parse time — the regex must compile and expose ≥2 capture groups, otherwise a helpful error.
• Zero-width-match guard so a loose pattern (e.g. (\w*)-(\d*)) can't loop forever.

Scope / non-goals

Wired through the add path (extractLinearIssueIdentifiersForCommit) only — the revert path (extractRevertedIssueIdentifiersForCommit) is intentionally left on the built-in magic-word gating to avoid false "reverted" attributions from arbitrary user patterns. Happy to extend it there if you'd prefer symmetry.

Implementation

• extractors.ts — matchCustomSubjectPattern() + an optional ExtractOptions arg on extractLinearIssueIdentifiersForCommit.
• args.ts — parse/validate --issue-pattern (compiles + ≥2 groups via a small countCapturingGroups probe).
• scan.ts / types.ts — thread issuePattern through ScanOptions and record it on the DebugSink.
• index.ts — help text, example, wiring.

Tests & docs

• extractors.test.ts — Conventional Commits with/without scope and !, case/leading-zero normalization, multi-ID subjects, subject-only (body ignored), de-dup with branch name, zero-width-loop guard.
• args.test.ts — default null, raw passthrough, empty = none, invalid-regex error, <2-groups error.
• scan.test.ts — extraction through scanCommits + debug-sink recording.
• README.md — new "Custom Issue Patterns" section + options-table row.

pnpm typecheck, pnpm test:ci (399 passing), pnpm lint (0 findings), and pnpm fmt:check all green locally.

[olsavmic]

Closing — this duplicates the earlier #71, which proposed the same --issue-id-pattern regex flag (capture group around the ID) and was closed unmerged. Closing the companion issue #106 too. Thanks!