feat: task dependency CRUD — create/update deps via CLI and API

[0pfleet]

Summary

Closes the #1 gap identified in the Sibyl audit: users cannot build dependency chains after task creation. The dependency reader (graph traversal, cycle detection, topological sorting via sibyl explore dependencies) was complete, but the writer was missing.

• PATCH /tasks/{task_id} now accepts add_depends_on / remove_depends_on for post-creation dependency management
• sibyl task create --depends-on task_aaa,task_bbb wires dependencies at creation time via the dedicated POST /tasks endpoint
• sibyl task update task_xxx --add-dep task_yyy --remove-dep task_zzz mutates dependencies after creation
• New RelationshipManager.delete_between() helper for targeted DEPENDS_ON edge removal in a single Cypher query

Motivation

Sibyl had a full dependency reader but an incomplete writer:

• POST /tasks accepted depends_on at creation, but the CLI never wired it (used generic POST /entities instead)
• PATCH /tasks had no dependency mutation support at all
• No way to add/remove dependencies after task creation

What changed

File	Change
sibyl_core/graph/relationships.py	Add delete_between() — single-query edge deletion between two nodes
sibyl/api/routes/tasks.py	Add add_depends_on/remove_depends_on to UpdateTaskRequest, handle in sync + async paths
sibyl/jobs/queue.py	Pass dep params through to worker
sibyl/jobs/entities.py	Handle dep mutations in async worker job
sibyl_cli/client.py	Add create_task() method (dedicated endpoint), add dep params to update_task()
sibyl_cli/task.py	Add --depends-on, --add-dep, --remove-dep flags; switch create to POST /tasks

Design decisions

• CLI create switched from POST /entities to POST /tasks: The generic endpoint queued a worker job that called create_direct() anyway (no LLM extraction for tasks). The dedicated endpoint does the same thing inline, eliminating the worker round-trip and pending state complexity. The --sync flag was removed as it's no longer needed.
• delete_between() returns count (not bool): Follows the pattern of delete_for_entity() — bulk-style operations return count, scalar operations (delete()) return bool.
• _build_update_data() extracted as helper: The PATCH handler hit PLR0915 (too many statements) after adding dep handling. Extracting the field mapping keeps it clean.

Test plan

• ruff check passes on all 3 packages (core, api, cli)
• ty check — no new type errors (11 pre-existing warnings in unrelated files)
• pytest sibyl-core — 769 passed
• pytest api — 1229 passed, 1 failed (pre-existing: test_tools_manage tries live FalkorDB connection)
• Manual smoke test after install:

  sibyl task create --title "Test dep" --depends-on task_aaa,task_bbb
  sibyl task update task_xxx --add-dep task_yyy
  sibyl task update task_xxx --remove-dep task_yyy
  sibyl explore dependencies task_xxx

🤖 Generated with Claude Code

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