GitHub Issues tracker provider

[andrewklingelhofer]

Summary

Adds a third issue-tracker provider, github, alongside Linear and Jira. Critters can now poll GitHub Issues in configured repos, claim them by status, run phases, and report outcomes back — using the newly-GA issue fields API for statuses on org repos, with automatic status:* label fallback on personal repos.

• src/tracker/github.ts — full IssueTracker implementation over plain fetch (no new deps, no GraphQL). Task identity is owner/repo#N everywhere. Status mode is detected per repo at init: org single-select issue field (default Status) when available, status:<Name> labels otherwise. Field values are set by option name via POST (never PUT, which would clobber other fields). updateStatus never throws. Rate-limit responses throw digit-free messages so isPermanentTrackerError classifies them as retryable. Blockers via the issue-dependencies API, gated on the embedded summary count.
• Webhook — POST /webhook/github reusing the shared HMAC verifier (X-Hub-Signature-256); signed issues events on configured repos with trigger labels kick a debounced single-issue poll. Closed issues never trigger. Dashboard hooks page + env-var list gain github entries (server-side only — no bundle regen).
• buildProviderConfig() — single ProviderConfig construction point; fixes three latent bugs where any non-jira provider silently became Linear (config-reload, cli-clean) and the daemon switch had no default case.
• Identifier safety — sanitizeIdentifier() (owner/repo#42 → owner-repo-42, no-op for Linear/Jira) applied to branch names, work dirs, plan/checkpoint paths (prompts + all runners), log/attachment names, the logs CLI, and logger regexes; /api/logs/<id> now URL-decodes.
• Config — github: block (repos, statusField, statusMap, statusTypes), GITHUB_TOKEN, GITHUB_WEBHOOK_SECRET, credential validation, statusType-bucket warning.

Notes

• Dual status mode and webhook inclusion were explicit design decisions; limitations (no attachment uploads, field-mode human edits discovered by polling, api.github.com only) are documented in the README's new "GitHub Issues as a ticket source" section.
• Reviewed with a spec-first pass (42 edge cases) and a deep review against the spec; all confirmed findings fixed (identifier casing canonicalization, closed-issue webhook guard, cli-clean prefix ordering, anchored HTTP-status classification, PATCH body hygiene).
• Two items need one live-API smoke check before production use (mocks can't settle them): that the issues list endpoint embeds issue_field_values, and that the ensureStatus options PATCH round-trips existing option colors.

Test plan

• bun test — 1009 pass / 0 fail (74 new cases: tracker modes/behaviors, webhook signature+extractor+endpoint, health logs decoding, config validation, utils sanitize, cli-clean identifier extraction)
• bun run typecheck
• bun run lint
• Live smoke against a real org repo (see Notes)

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