Add delegation timestamps and refresh worktree hints

[dmoliveira]

Summary

• add a delegated_at timestamp to delegated task focus headers so hook/plugin-based delegation carries visible timing context
• prevent WORKTREE CONTEXT headers from accumulating when agent-model-resolver reshapes a delegation across worktrees
• extend gateway-core hook tests to cover timestamp injection and rerouted worktree context replacement

Validation

• npm exec --yes eslint src/hooks/agent-context-shaper/index.ts
• npm run build && node --test test/agent-context-shaper-hook.test.mjs
• npm exec --yes eslint src/hooks/agent-model-resolver/index.ts test/agent-model-resolver-hook.test.mjs
• npm run build && node --test test/agent-model-resolver-hook.test.mjs