[DSP-2] Planning tools: decompose_feature / get_project_state / get_next_task / prioritize_issues from devswarm

[justrach]

Child of #53. Source: justrach/devswarm.

Tools to port (4)

DevSwarm tool	Purpose	Forge landing zone (proposed)
decompose_feature	Break a feature spec into sub-tasks (filed as linked issues by default)	forge_app planner + SDK planning.decompose
get_project_state	Snapshot of repo: branches, open PRs, recent changes, hot files	forge_app + uses git tools from DSP-4
get_next_task	Pick highest-priority work item from open issues	forge_app planner
prioritize_issues	Rank a set of issues by impact / dependency / staleness	forge_app planner

Acceptance criteria

• decompose_feature produces a parent + child issue tree on this repo, using the existing label vocabulary (priority:p0–p2, type:, work:, severity:*).
• Issue body template enforces a repro/evidence gate for bug-class items — one-sentence problem, repro steps, observed/expected, acceptance criteria, non-goals (devswarm #394 was the lesson here).
• create_issue parent_issue= actually links the subtask (devswarm shipped this broken, their #393) — use GitHub task-list syntax in the parent body or sub-issues API.
• get_project_state returns structured JSON consumable by the orchestrator from DSP-1.

Related codegraff issues

• [P4-11] graff conversation export <cid> (JSONL) + graff doctor #45 [P4-11] graff doctor — get_project_state is the same idea, may want to consolidate.