docs(ci): document cargo-binstall/tarpaulin usage in rust check job

Add workflow comment explaining why the rust job installs
cargo-binstall and cargo-tarpaulin after mode: check setup.
Close TASK-320 — audit confirms no unused CI tool installations
remain after TASK-317's check/full split.

Author: pythoninthegrass

.github/workflows/test.yml

@@ -57,6 +57,9 @@ jobs:
       - name: Cargo clippy
         run: cargo clippy --workspace --all-features -- -D warnings
 
+      # cargo-binstall and cargo-tarpaulin are installed separately from the
+      # composite action (which runs in check mode and skips cargo tools).
+      # These are needed only for coverage — not for fmt, clippy, or check.
       - uses: cargo-bins/cargo-binstall@main
 
       - name: Install cargo-tarpaulin

backlog/tasks/task-320 - Stop-installing-unused-CI-tools-in-non-build-jobs.md

@@ -1,9 +1,10 @@
 ---
 id: TASK-320
 title: Stop installing unused CI tools in non-build jobs
-status: To Do
+status: Done
 assignee: []
 created_date: '2026-04-12 06:59'
+updated_date: '2026-04-12 09:00'
 labels:
   - ci
   - github-actions
@@ -29,8 +30,42 @@ Trim CI setup further by ensuring jobs only install cargo and frontend tooling t
 
 ## Acceptance Criteria
 <!-- AC:BEGIN -->
-- [ ] #1 Non-build CI jobs do not install cargo or frontend tools that they never invoke.
-- [ ] #2 Any job that still installs extra tools has a documented reason in workflow comments, task notes, or related docs.
-- [ ] #3 The change does not remove required tooling from release, bundle, or full-build jobs.
-- [ ] #4 The reduced installation scope is validated with the affected workflow jobs or an equivalent local/static verification step.
+- [x] #1 Non-build CI jobs do not install cargo or frontend tools that they never invoke.
+- [x] #2 Any job that still installs extra tools has a documented reason in workflow comments, task notes, or related docs.
+- [x] #3 The change does not remove required tooling from release, bundle, or full-build jobs.
+- [x] #4 The reduced installation scope is validated with the affected workflow jobs or an equivalent local/static verification step.
 <!-- AC:END -->
+
+## Implementation Plan
+
+<!-- SECTION:PLAN:BEGIN -->
+## Implementation Plan
+
+### Analysis Result
+TASK-317's `mode: check` split already eliminated the main waste. After review, no non-build CI jobs install cargo or frontend tools they never invoke. The only "extra" installs are in the `rust` job (cargo-binstall + cargo-tarpaulin), which are legitimately needed for coverage.
+
+### Steps
+1. Add workflow comments to `test.yml` `rust` job documenting why cargo-binstall and cargo-tarpaulin are installed after check-mode setup
+2. Validate with actionlint
+3. Mark ACs as met with notes
+<!-- SECTION:PLAN:END -->
+
+## Implementation Notes
+
+<!-- SECTION:NOTES:BEGIN -->
+## Audit Results (2026-04-12)
+
+After TASK-317 introduced `mode: check` vs `mode: full`, all non-build CI jobs already avoid installing unused tools:
+
+- **rust** (test.yml): uses `mode: check` (skips Node.js, npm ci, cargo-binstall via composite, tauri-cli). Installs cargo-binstall + cargo-tarpaulin separately — justified by coverage step. Comment added to workflow.
+- **deno-lint** (test.yml): installs only Deno. No Rust/cargo/frontend tooling.
+- **build macOS/Win** (test.yml): uses `mode: check`. No extra tools.
+- **build Linux** (test.yml): Docker-based, no composite action.
+- **vitest-tests** (test.yml): Node.js only.
+- **playwright-tests** (test.yml): Node.js only.
+- **release jobs** (release.yml): use `mode: full` (default) — correct for build+bundle.
+
+No unnecessary tool installations found. Added documentation comment to the `rust` job explaining the cargo-binstall/tarpaulin installs.
+
+Validated with `actionlint` — only pre-existing shellcheck warnings in release.yml, no new issues.
+<!-- SECTION:NOTES:END -->