feat: label generated PRs via pr_labels (workspace.toml) and PR_LABELS env

[danii1]

Summary

• Add pr_labels to workspace.toml ([defaults] fleet-wide default, per-[[repos]] override) so fleet-created PRs get labeled automatically
• buildRepoEnv injects the resolved labels as PR_LABELS into every task/automation subprocess for that repo
• PRManager picks up PR_LABELS (comma-separated) — so single-repo CLI users can also set it in .devintern-code/.env
• GitHubPRClient applies labels via POST /repos/{owner}/{repo}/issues/{n}/labels after creation and on the 422 already-exists idempotent path; best-effort (warns, never fails the run). GitHub only — Bitbucket has no PR label API

Notes

• Deliberately no CLI flag: labels are automation/fleet policy, so they live in workspace.toml / env
• Settings.json untouched (tracker-side state stays there; PR labels are repo-side)
• workspace init scaffold documents the new key

Testing

• bun run test: 1018 pass (new tests for config parsing/inheritance/validation, buildRepoEnv injection + precedence, label application incl. 422 path and best-effort failure)
• format / lint / typecheck green via lefthook