test(#804): reproduce unknown installer argument

docs/scratchpads/804-install-unknown-flags.md

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+# Issue #804 — fail closed on unknown installer arguments
+
+## Objective
+
+Implement Part 1 of Gitea issue #804 only: `tools/install.sh` must reject every unrecognized flag or argument with an actionable STDERR error and nonzero exit before installation starts.
+
+## Scope and constraints
+
+- Preserve all currently recognized options and behavior, including `-y` and `--ref <branch>`.
+- No positional arguments are currently accepted by the parser.
+- Do not add `--next`, `MOSAIC_NEXT`, prerelease routing, or any Part 2 behavior.
+- TDD is mandatory: add and observe a failing process-level regression test before changing `tools/install.sh`.
+- Worker lifecycle ends after branch push, PR creation, and coordinator notification; do not merge or close #804.
+- Existing launcher-owned changes in `.mosaic/orchestrator/mission.json` and `.mosaic/orchestrator/session.lock` are out of scope and must not be committed.
+
+## Requirements and acceptance criteria
+
+- Unknown input names the offending argument on STDERR.
+- STDERR includes a short installer usage hint.
+- Exit status is nonzero.
+- The installer does not invoke npm or otherwise proceed into installation.
+- Existing recognized flags remain unchanged.
+
+## Plan
+
+1. Add a process-level Vitest regression using the installer test location under `packages/mosaic/src/commands/`.
+2. Run the focused test and record the expected RED failure.
+3. Commit the RED test as `test(#804): ...`.
+4. Replace the parser catch-all with a fail-closed STDERR error and usage hint.
+5. Update concise installer-facing documentation without introducing prerelease behavior.
+6. Run focused tests, shell syntax validation, package tests, lint, typecheck, and format checks.
+7. Run independent review tooling and remediate findings.
+8. Commit as `fix(#804): ...`, queue-guard, push, open a PR containing `Closes #804.`, notify the coordinator, and exit.
+
+## Budget
+
+- No explicit token cap supplied.
+- Working estimate: 8K tokens; narrow two-file behavior/test change plus concise docs and delivery gates.
+
+## Progress
+
+- 2026-07-17: Loaded mission state, issue #804, delivery/QA/documentation rails, and relevant TDD/Vitest/pnpm/Gitea skills.
+- 2026-07-17: Confirmed the parser has no legitimate positional arguments and currently drops all unmatched input via `*) shift ;;`.
+- 2026-07-17: Installed locked workspace dependencies with a worktree-local pnpm store; no lockfile changes.
+- 2026-07-17: Added the process-level unknown-argument regression with an isolated `$HOME` and npm shim.
+
+## Verification
+
+- RED: `pnpm --filter @mosaicstack/mosaic exec vitest run src/commands/install-arguments.spec.ts` — expected failure: installer exited `0` instead of nonzero at the exit-status assertion; confirms the test reproduces the silent-drop defect before production changes.
+
+## Risks and blockers
+
+- Part 2 remains owner-gated under #805 and is intentionally excluded.
+- Test harness must isolate `$HOME` and shim npm so the pre-fix installer cannot mutate operator state or contact the registry.