t.18h: filter terminal refs in assignment create

[cs3b]

📋 Summary

Reviewers and operators can now trust work-on-task assignment creation to handle terminal task refs cleanly, including no-oping when every requested ref is already terminal. Before this, terminal refs could leak into prepare/create flows and create avoidable assignment-generation churn for all-terminal requests. This update tightens the prepare/create contracts, aligns user-facing messaging/docs, and ships regression coverage so mixed/all-terminal inputs are handled deterministically.

✏️ Changes

• Updated assign/prepare terminal filtering contract: resolves requested refs first, filters done/skipped/cancelled, continues mixed sets, and aborts all-terminal sets with explicit no-assignment messaging.
• Added assign/create all-terminal guardrails: blocks hidden-spec render and ace-assign create when prepare returns only terminal refs; clarifies mixed-ref behavior uses filtered refs.
• Synced usage and workflow docs: documents terminal-status handling and user-visible outcomes for mixed/all-terminal work-on-task inputs.
• Added WorkOnTaskTerminalFilterContractTest coverage: validates prepare/create wording and usage docs against the shared terminal-status contract.
• Finalized release artifacts: bumped ace-assign to 0.38.2 and recorded project/package changelog entries for the shipped increments.

📁 File Changes

 +237,    -7    9 files   total

 +183,    -6    6 files      ace-assign/
   +1,    -1    1 files   🧱 lib/
   +1,    -1                 ace/assign/version.rb
  +70,    -0    1 files   🧪 test/
  +70,    -0                 organisms/work_on_task_terminal_filter_contract_test.rb
  +75,    -3    2 files   📚 handbook/
  +14,    -3                 workflow-instructions/assign/create.wf.md
  +61,    -0                 .../prepare.wf.md
  +37,    -2    2 files      
  +24,    -0                 CHANGELOG.md
  +13,    -2                 docs/usage.md

   +1,    -1    1 files      .ace-tasks/
   +1,    -1                 8qp.t.18h-filter-done-tasks-during-assignment/8qp.t.18h-filter-work-on-task-inputs.s.md

  +35,    -0    1 files      .ace-retros/
  +35,    -0                 8qp20k-t-18h-done-task-filtering/8qp20k-t-18h-done-task-filtering-assignment.retro.md

  +18,    -0    1 files      ./
  +18,    -0                 CHANGELOG.md

🧪 Test Evidence

• WorkOnTaskTerminalFilterContractTest — validates terminal-status filtering language, mixed/all-terminal examples, create-flow all-terminal abort behavior, and usage-doc contract alignment.
• ace-test ace-assign — package suite passed with 484 tests, 1539 assertions, 0 failures, 0 errors in 5.63s.
• ace-assign layer totals — atoms 207, molecules 66, organisms 51, models 76, commands 84 (all passing).

📦 Releases

• ace-assign v0.38.0 — introduced explicit work-on-task done-task filtering guidance in prepare/create workflows and docs.
• ace-assign v0.38.1 — added regression coverage for mixed/all-terminal filtering and create-flow no-op behavior.
• ace-assign v0.38.2 — aligned contracts to terminal statuses (done/skipped/cancelled) and moved contract coverage into test/organisms/.
• project v0.9.926–0.9.928 — recorded coordinated root changelog releases corresponding to the above ace-assign increments.