feat(bus): add --blocked-by to update-task for post-creation blockers

[asachs01]

Summary

• task_1786923653812_76952898 / task_1786773702893_68177835 (two independent filings of the same gap, grower + boss): update-task only accepted --assignee/--project — there was no way to attach a blocked_by edge to a task after creation, even though a task normally transitions to blocked status after it already exists (that's what triggers the transition in the first place). The only workaround was an unstructured log-event ... --meta blocked_by:... note that no tooling reads.
• Concrete evidence cited in the filings: check-stale-blockers scanning 100+ blocked tasks a night and finding almost none of them checkable, because they carry no blocked_by field at all — this is also what motivated fix(bus): add coverage denominator to checkStaleBlockers summary #140 (coverage denominator), merged/pending ahead of this one per the filing's own ordering argument.
• Adds --blocked-by <ids> to update-task (comma-separated), and a matching blockedBy?: string[] option on updateTask(). Semantics: additive (appends to the existing list, never replaces it — dedupes ids already present), cycle-checked the same way create-task is (rejects before any write, so a rejected cycle leaves zero partial state), and writes the same symmetric blocks edge on the peer task that create-task does.

Test plan

• npx vitest run tests/unit/bus/task.test.ts — 71/71 passing, including 8 new tests: basic add, additive-not-replace, dedup, cycle rejection (both a general loop and self-reference), the zero-partial-state regression shape (mirroring the existing createTask cycle regression test), audit log entry, and the exact motivating scenario end-to-end (blocked task with no blocked_by → attach after the fact → checkTaskDependencies can now evaluate it)
• Updated 2 pre-existing tests whose exact error-message strings changed (task.test.ts, bus-task-error-handling-cli.test.ts)
• npx tsc --noEmit — clean
• Full suite regression check — 6 pre-existing failures, all in phase5-performance.test.ts / cron-scheduler.test.ts (timing/timeout under load), matching this repo's documented flaky-under-load pattern; none touch src/bus/task.ts or src/cli/bus.ts

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