feat(gateway): G8 part b — typed blocker_type + what_needed on i_am_blocked

Pre-gateway parity (G8 part b of the 2026-05-11 design). The pre-gateway
TaskBlockInput at 254cc93:roboco/mcp/schemas/__init__.py required
blocker_type (external|internal|question|dependency) and what_needed
so PMs could triage their inbox by class. Current i_am_blocked dropped
both fields — every blocked task looked the same to the PM.

Now i_am_blocked accepts both as optional kwargs:
- Back-compat: callers that omit them still work (blocker_type defaults
  to None → rendered as flat reason in the struggle entry).
- New: when supplied, the struggle journal entry body is structured
  markdown (## Blocker Type / ## What Needed sections) so the panel's
  journal view renders named blocks instead of one flat sentence.

Validator on blocker_type enforces the enum at the Pydantic boundary
with a clear "must be one of: ..." error if the agent invents a value
(same pattern as the Wave 3 G7 validators).

G8 part a — typed `pause(checkpoint_summary, remaining_work)` — defers.
That gap needs a new IntentSpec in foundation/policy/lifecycle.py
(currently pause is an ActionSpec only; agents auto-pause via i_am_idle)
plus checkpoint wiring through TaskService.add_checkpoint. Material
work, deferred until after the user has deployed and verified G7 +
G8b lands cleanly.

Wired:
- roboco/api/schemas/v2/flow.py — IAmBlockedRequest gains optional
  blocker_type + what_needed; @field_validator enforces the enum
- roboco/api/routes/v2/flow_dev.py — passes the new fields through
- roboco/services/gateway/choreographer/_impl.py — i_am_blocked signature
  + structured struggle-entry rendering
- roboco/mcp/flow_server.py — typed wrapper with the kwargs
- agents/prompts/roles/developer.md — updated verb table
- tests/unit/mcp_servers/test_flow_server.py — updated to expect the
  new optional kwargs as None when omitted

Quality: ruff + mypy clean. 505 tests pass.

agents/prompts/roles/developer.md

@@ -22,7 +22,7 @@ You write code; you do not coordinate. If you find yourself thinking "let me als
 | `commit(message)` | Makes the git commit, auto-prefixes `[task-id]`, records a progress entry. This is the ONLY way to commit — the gateway covers the actual git operation. | Task in `in_progress`; on your branch. |
 | `open_pr(task_id)` | Push your branch and open a PR. Run after your last commit, before `i_am_done`. | Task assigned to you; at least one commit; no PR yet. |
 | `i_am_done(task_id, notes)` | Submit for QA. Auto-runs in_progress→verifying→awaiting_qa. Requires PR already open — run `open_pr` first. | At least one commit; PR open; progress entry; journal `reflect`; every acceptance criterion addressed. |
-| `i_am_blocked(reason)` | Records the blocker, escalates to your PM, idles you. | Task is yours and active. |
+| `i_am_blocked(task_id, reason, blocker_type?, what_needed?)` | Records the blocker, escalates to your PM, idles you. `blocker_type` ∈ `external` (waiting on a 3rd-party API/service), `internal` (a teammate or process), `question` (need clarification), `dependency` (waiting on another task). `what_needed` is a one-sentence concrete unblock request. Both fields are pre-gateway parity — PMs triage by class. | Task is yours and active. |
 | `unclaim(task_id)` | Release this claim back to pending. Use sparingly — your work-in-progress branch survives but the task is unassigned. | Task assigned to you and in claimed/in_progress. |
 | `resume(task_id)` | Resume a paused task. Transitions paused → in_progress. | Task assigned to you and in paused state. |
 | `note(text, scope?)` | Journal entry (`scope ∈ note|decision|reflect|learning|struggle`). | None. |