[#64] Add GrantFox issue batch quality validator

[Alimzy]

Summary

Adds a local validator for GrantFox-style advanced issue JSON batches, catching missing fields, unsupported labels, missing ISSUE_STANDARD.md sections, and weak acceptance criteria before any issue is created on GitHub.

Closes #64

Acceptance criteria

• Issue batch schema validator is implemented -- scripts/validate-issue-batch.mts, validateIssueBatch()
• Unsupported labels are detected before GitHub issue creation -- checkLabels() cross-references .github/LABELS.yml
• Missing required fields are reported clearly -- checkRequiredFields(), one violation per missing field with entry index/title
• Weak acceptance criteria are flagged -- checkBodyStandard(), WEAK_ACCEPTANCE_PHRASES list
• Tests cover valid and invalid issue batches -- scripts/validate-issue-batch.test.ts, 6 tests
• Documentation explains the advanced issue standard -- docs/ISSUE_BATCH_VALIDATOR.md

Design notes

• Placed as a standalone script (scripts/validate-issue-batch.mts), matching the existing scripts/check-package-boundaries.mts convention (dependency-free, runs via tsx, gates before pnpm install even resolves workspace packages) -- not under packages/validators, since that package's own boundary rules (and its actual purpose: Stellar/anchor domain schemas) make it the wrong home for GrantFox meta-tooling.
• LABELS.yml is hand-parsed (no new yaml dependency) since its format is a simple flat list and the repo has no existing yaml parser dependency.
• The "campaign issue" checks (label taxonomy, required sections, weak acceptance criteria) only apply to entries carrying all three campaign labels (GrantFox OSS, Maybe Rewarded, Official Campaign | FWC26), per ISSUE_STANDARD.md's own scope ("Anything less is still a valid GitHub issue").

Verification

• pnpm validate:issues examples/issue-batches/valid-batch.json -- exit 0
• pnpm validate:issues examples/issue-batches/invalid-batch.json -- exit 1, 16 violations reported clearly
• The invalid fixture is modeled on this issue's own real-world labels (monorepo, feature, developer-experience) -- none of which exist in .github/LABELS.yml. Running the validator against them catches exactly this class of mistake, the same one AUTOMATION_RUNBOOK.md's troubleshooting table already names as the most common batch failure.
• npx vitest run scripts/validate-issue-batch.test.ts -- 6/6 passing
• pnpm typecheck -- 12/12 successful
• pnpm lint -- clean