fix: make work item state transitions mandatory and explicit

[fedeoliv]

Problem

The implement coordinator and the devsquad.implement.finalize worker treated board updates as advisory. Result observed in a recent session: GitHub issues and Azure DevOps work items stayed in New through the full implement -> PR cycle even though every required tool (ado/wit_update_work_item, github/issue_write) was wired up.

The agent acknowledged the gap directly when asked: "The work-item-workflow skill exists exactly for this and I haven't been invoking it." So this is not a knowledge gap. It is an invocation and enforcement gap, and it recurs across sessions.

Two reinforcing causes:

1. Source classification weakness. The work-item-workflow skill's source-detection rule treated "no explicit ID in this message" as tasks.md only, even when a work item ID was present in the recent conversation or the tasks.md being implemented.
2. Soft language at the finalize step. devsquad.implement.finalize instructed "Update the work item state as appropriate" with no state machine, no preconditions, and no observable output the coordinator could check.

Changes

• skills/work-item-workflow/SKILL.md: Source detection now resolves to board mode whenever a work item ID is reachable from the user's request, recent conversation, or the tasks.md being implemented (e.g., "implement the Terraform task" with #1690 upthread). Phase 2 makes state transitions mandatory, with a re-read confirmation and a single retry before surfacing failure. If the board MCP is unreachable, the agent surfaces a blocking error rather than skipping.
• agents/devsquad.implement.agent.md: Phase 1 documented as a precondition. No code-editing tool runs until the task and parent user story are Active with the current user as assignee. Added a Notes rule reaffirming that transitions are mandatory and that MCP failures must surface, not be skipped.
• agents/devsquad.implement.finalize.agent.md: "Update as appropriate" replaced with an explicit per-platform state machine (Active on start -> Resolved on PR open -> Closed on merge), procedural steps that use ado/wit_get_work_item to verify preconditions, and a structured Board Updated: #<id> <prev> -> <new> output line so the coordinator can detect skipped transitions.

Out of Scope

• No hook. The instruction-level fix should be measured first. If the gap recurs after these changes ship, a deterministic hook becomes the next iteration with stronger justification.
• No harness-learnings entry. This is a framework gap that affects every consumer, not a per-codebase pattern. The skill itself excludes "learnings about the framework" from capture (SKILL.md line 39).
• No automatic close on merge. The existing rule stands: PR uses Closes #N for GitHub; ADO transition to Closed remains the developer's action.