feat(opencode): one-task-per-run model + 9 routing fixes (ADRs 13-21)

Captures the design grilling outcome. Adds ADRs 13-21 covering:
- run-level plan_rework_remaining counter to bound P3<->P5.5/P7/P8 thrash
- non-resumable workflow with throwaway-worktree recovery procedure
- @simplify advisory at every gate (not just Phase 8)
- Phase 8 fix specs go to disk as task-fix-N.md (preserves ADR-6)
- Phase 5.5 BLOCK protocol: orchestrator edits plan, decrements counter, re-enters P4
- Phase 8 NOT_TESTABLE manifest in reviewer prompt
- unified Implementation Incomplete diagnosis (test_design / production_logic / split_needed)
- Phase 1 working-tree cleanliness + depends-on enforcement
- one-task-per-run pivot: Phase 5 still splits N tasks, only task-1 runs;
  tasks 2..N filed as sub-issues with rich seed bodies; split_needed at P7
  aborts to Failure Handler (one-task-per-run = no salvageable prior work)

Auto-resolves big-diff Phase 8 reviews, cross-task regression-within-run, and
mid-flight task-split routing. Rewrites routing matrix and three Mermaid
diagrams; updates @pm (depends-on frontmatter, split-time filing), @check
(third diagnosis verdict), @make (escalate: split_needed flag).

config/opencode/agents/pm.md

@@ -48,6 +48,7 @@ title: Implement a special stage type
 status: Done
 parent: GAL-38
 labels: [gameplay, advanced-mechanics]
+depends-on: [GAL-37]
 ---
 
 # GAL-39: Implement a special stage type
@@ -80,6 +81,7 @@ Free-form markdown describing the problem and context. Spans as many paragraphs
 - `status` — one of: `Todo`, `In Progress`, `Done`. (No other values; the old `Backlog`/`In Review`/`Cancelled` set is gone.)
 - `parent` — either `null` (top-level issue) or another issue ID (e.g. `GAL-38`). Sub-issues belong to their parent's `## Sub-issues` list.
 - `labels` — YAML list of strings, e.g. `[gameplay, advanced-mechanics]`. May be `[]`.
+- `depends-on` — *optional* YAML list of issue IDs that must reach `status: Done` before this issue can be started. Used by `/workflow`'s Phase 1 sanity check to hard-block runs whose dependencies aren't satisfied (per ADR-21). Omit the field entirely when there are no dependencies; do not write `depends-on: []`. Cycles are not detected by this agent — the caller is responsible for not creating a cycle.
 
 **Body rules:**
 - The first heading is `# <ID>: <title>` (matches frontmatter).
@@ -172,9 +174,24 @@ For list output, return an array of `{id, title, status, parent, labels}` object
 ### When creating issues
 - Default `status: Todo` unless the caller says otherwise.
 - Title: short, imperative ("Add retry logic to ingest worker", not "retry stuff").
-- Frontmatter must be complete: `id`, `title`, `status`, `parent`, `labels`.
+- Frontmatter must be complete: `id`, `title`, `status`, `parent`, `labels`. Add `depends-on:` when the caller specifies dependencies.
 - Always update the dependent index (README.md for top-level, parent file for sub-issues) so the new issue is visible.
 
+### Split-time sub-issue creation (rich-body filings)
+
+When the `/workflow` orchestrator dispatches you mid-run to file a sub-issue from a Phase 5.5 task split (per ADR-21), the caller passes a structured body containing more than the usual minimum. Treat the body as already-finalized — write it verbatim into the new issue file. Common sections you'll see:
+
+- `## What to implement` — one-line + brief description.
+- `## Acceptance criteria` — checkboxes; preserve `- [ ]` state (newly filed sub-issues start with all AC unchecked).
+- `## Code Context` — code snippets carried over from the split-time task spec.
+- `## Integration with sibling sub-issues` — narrative; the structural dependencies belong in the `depends-on:` frontmatter list, which the caller will pass alongside the body.
+- `## Plan rationale` — slice of the parent's plan.
+- `## Test design` — when present.
+
+Use the rendered ordering: H1 → description (the "Discovered during run on …" attribution paragraph that ends the body counts as part of the description) → `## Sub-issues` (omit; sub-issues won't have their own children at filing time) → `## Acceptance criteria` → `## Integration test hints` (omit unless caller passed it) → `## Comments` (omit until first comment is appended).
+
+Add the `split-from-run` label to the labels list when the caller specifies it, alongside any propagated parent labels.
+
 ### When updating status
 - Confirm the change (e.g. "GAL-39 status: In Progress → Done").
 - A status change to `Done` is only valid if all acceptance-criteria checkboxes (when the section exists) are checked. If they are not, report which ones remain and ask for confirmation before forcing the change.