Fix /agent refine no-op for non-gated tasks#

Problem#

/agent refine was falling through to the generic directive handler, which dispatched an event and posted a misleading confirmation comment even when the task was not in a gated state (where refine has no effect).

Task / Link#

Closes #233

Changes#

• Added refine pre-validation block in DirectiveService.applyDirective() before the generic handler
• Terminal tasks (succeeded/failed): post rejection comment, no dispatch/enqueue
• Non-gated tasks: post rejection comment with current status, no dispatch/enqueue
• Gated tasks only: clear gate, save, enqueue refine phase, post confirmation
• Replaced single refine test with describe('refine directive') block covering 5 scenarios

Notes#

• task.directive is not overwritten — preserves auto/careful/quick through the refine flow
• clearGate is called before enqueuePhase, matching the existing retry/restart pattern

Testing#

• Unit tests: new describe('refine directive') block with 5 tests (gated ✓, active ✗, succeeded ✗, failed ✗, directive-preserved ✓)
• Full suite: 665 tests pass
• Lint: zero errors in changed files
• Build: compiles cleanly