feat(output): add number and context outputs, soft-deprecate issue_number

[wadackel]

Summary

Closes #100.

• Add number output that mirrors issue_number and is named generically for both issue and pull_request contexts.
• Add context output that emits "issue" or "pull_request", removing the need for downstream steps to recompute context.payload.issue.pull_request != null.
• Prefix the issue_number description with [Deprecated] and recommend number (documentation-only soft deprecation; the field keeps emitting in v1 and is scheduled for removal in the next major release).
• Lift the isPr calculation to a single site in run() and pass it into isValidContext, so the validity check and the new context output share one source of truth.
• Split src/index.ts out as the thin runner that bridges run() to process.exit, and keep src/main.ts side-effect-free so it can be imported from src/main.test.ts without firing the top-level exit path. package.json build script and AGENTS.md entrypoint docs updated accordingly.
• Add src/main.test.ts with vi.mock based coverage for issue context, pull_request context, and invalid context (none of number / context / issue_number emitted).

Acceptance Criteria (from #100)

• action.yaml declares number and context outputs.
• action.yaml description of issue_number is prefixed with [Deprecated] and recommends number.
• src/main.ts emits number, context, and continues to emit issue_number with the same value.
• number === issue_number for both issue and pull_request contexts.
• context === "pull_request" when the comment is on a PR, otherwise "issue".
• When isValidContext returns false, none of number / context / issue_number are emitted.
• pnpm build produces an updated dist/, committed in the same PR.
• pnpm generate is run (README Outputs table regenerated).
• Tests cover issue context, pull_request context, and invalid context.
• README Outputs table reflects the new fields and the deprecation note.

Follow-up (out of scope, tracked separately)

• Harden isValidContext post-condition so the action does not throw before emitting continue=false on malformed issue_comment payloads (currently relies on GitHub-guaranteed payload shape).
• Regression test for comment.user = null.
• Coverage for allowed_contexts filtering interaction with the new context output.
• Physical removal of issue_number (next major release; separate issue/milestone).

References

• closes feat(output): add number and context outputs, soft-deprecate issue_number #100

[github-actions]

🎉 This PR is included in version 2.1.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀