feat: allow up to eight parallel GitHub tasks

[0xroylee]

What Changed

• Allow scheduler run --concurrency to select 1–8 independent GitHub tasks, retaining the default of 2.
• Validate the same bounds in the CLI and GitHubTaskPool.
• Add a gated nine-Issue regression proving eight slots fill, a ninth waits, and a released slot refills while unrelated tasks remain blocked.
• Update the CLI documentation and parallel-execution specification.

Why

The requested configurable parallel execution must use the current GitHub-native scheduler. This branch is reconciled with main's native worktrees, task checkpoints, scope admission, recovery, and guarded automatic merge. The superseded SQLite scheduler changes are removed from the final diff.

How to Test

1. Run bun run typecheck and bun run lint; both must pass.
2. Run bun run test; the complete suite must pass.
3. Run bun dev scheduler run --help; concurrency must show 1–8 with default 2.
4. Use --concurrency 0, 9, or a fraction; the CLI must reject the argument before execution.

Acceptance checklist

• Select 1–8 task slots while preserving the default of 2. Evidence: CLI boundary tests and help output.
• Do not exceed capacity; refill a freed slot without waiting for unrelated tasks. Evidence: gated eight-slot Fake Harness regression in test/scheduler/github-pool.test.ts.
• Preserve native worktrees, scope/dependency admission, cancellation, and serialized merge behavior. Evidence: main's implementation is retained and its regression suite passes.
• Human acceptance after checking the criteria, evidence, and limitations.

Additional Notes

The added eight-slot coverage is deterministic; no live eight-provider run is claimed. Conflicting or ambiguous scopes remain exclusive even when spare slots exist. Auto-merge remains opt-in and is serialized independently of task concurrency. No application PR was merged during this conflict resolution.