fix(branch): flush staged twig state transitions so workflow reads see fresh ADO (AB#3126)

[PolyphonyRequiem]

Summary

Fixes AB#3126 -- primary_completer failing with ""must be in InProgress state category, but is in Proposed"" in dogfood Run #7.

Root cause (corrected)

The original AB#3126 issue body claimed ""no upstream step transitions the task from Proposed to InProgress"". That was wrong. branch next-impl ALREADY does the begin_implementation transition via twig.SetStateAsync -- the actual bug is that the staged change never reached ADO before primary_completer's polyphony validate ran.

twig state only stages locally; without twig sync the change sits in twig's pending queue. ValidateCommand reads via IWorkItemRepository without syncing, so it saw stale Proposed and refused implementation_complete. Empirically: failure timestamp 14:12:28; ADO state-change timestamp on the same item 14:17 (after my manual sync intervention while filing the issue).

Changes

Verb-level (BranchCommands)

• branch next-impl -- post-state SyncAsync so the staged transition is durable before return.
• branch close-scope -- batched post-loop SyncAsync (guarded by closed.Count > 0 so an empty loop doesn't push).

Workflow YAML (implement-merge-group.yaml)

• primary_completer script: twig sync BEFORE polyphony validate (fresh read), twig sync AFTER twig state (durable write), plus ErrorActionPreference = Stop and PSNativeCommandUseErrorActionPreference = true so a failed sync halts rather than silently propagating stale state.
• primary_router: explicit error route to a new primary_router_error_gate BEFORE the catch-all. Without this, a failed next-impl would route error through the catch-all to dependency_check, silently treating the failure as ""all items done"" and skipping every remaining task -- a separate latent bug surfaced by rubber-duck critique.
• New primary_router_error_gate (human_gate, modeled on mg_pr_failed_gate_ado) with retry / abort options.

Convention

.github/skills/polyphony-cli-developer/SKILL.md gains a ""State-mutation durability -- when ADO must hold transition X"" section, sister-pattern to the existing ""Post-condition verification"" section. Verbs that mutate state via ITwigClient.SetStateAsync / PatchFieldsAsync and whose callers need that mutation to be visible to a different process MUST flush via SyncAsync before returning.

Tests

Three new regression tests using runner.Invocations ordering assertions:

• NextImpl_HappyPath_FlushesStagedTransitionAfterSetState
• CloseScope_BatchedTransitions_FlushesOnceAfterLoop
• CloseScope_NoTransitions_DoesNotFlush

Full Polyphony.Tests suite: 3390/3394 pass (4 pre-existing skips). All lints green (jinja-resolver, prose-children, lifecycle-router-coverage, type-agnostic, no-tracked-state). conductor validate confirms the workflow.

Anti-fix

The original issue body proposed adding a primary_starter workflow node before impl_branch_ensure. That would mask the underlying durability bug rather than fix it, and would leave every other staged-but-unflushed mutation site silently broken. The fix belongs at the C# verb layer, with the workflow YAML carrying complementary twig sync calls in script nodes.

Out of scope (filed as follow-ups)

Other unflushed mutation sites discovered during review -- same bug shape, deferred for surgical scope:

• AB#3127 -- ScopeCommands.TagMutationAsync
• AB#3128 -- PlanCommands.SeedChildren parent tag/facet
• AB#3129 -- apex-driver.yaml close_mark_satisfied + apex-item-dispatch.yaml terminal_satisfied workflow scripts
• AB#3130 -- Polyphony.csproj relative ProjectReference to twig2 fails in per-run worktrees (filed under AB#3085)

Sibling PR

Parent fork session is working on AB#3125 (scope-review deadlock) which also touches implement-merge-group.yaml -- but in the scope-review section (~line 832), not near the changes here (lines 240, 591-619, and the new gate insertion). Should auto-merge.

Verification

• dotnet build clean
• dotnet test -- 3390 passed
• conductor validate -- passes
• All lints green
• Rubber-duck critique applied (added explicit error route + pre-validate sync)
• AB#3126 description updated in ADO with corrected analysis

AB#3126