test(plugins): isolate plugin dir per test process

[woodsonl]

Summary

tests/_setup/isolateDataDir.ts now points OMNIROUTE_PLUGINS_DIR at $DATA_DIR/plugins in every test process. The plugin scanner stops reading the developer's real ~/.omniroute/plugins during local test runs, and the chat pipeline stops loading plugins from it.

Why

• getDefaultPluginDir() (src/lib/plugins/scanner.ts:46-57) returns OMNIROUTE_PLUGINS_DIR, else $HOME/.omniroute/plugins. The setup isolated DATA_DIR and left the plugin dir on the HOME default.
• So every chat-pipeline test (the 28 tests/unit files that use tests/integration/_chatPipelineHarness.ts) scanned the real plugin dir. A request-mutating plugin installed locally could turn them red, and its code ran inside the test processes.
• tests/unit/plugins-fs-safety.test.ts installed plugins into, and deleted them from, that same real directory.
• CI runners have a clean HOME, so CI never hit this.

Changes

• tests/_setup/isolateDataDir.ts: after the DATA_DIR block, default OMNIROUTE_PLUGINS_DIR to $DATA_DIR/plugins. A value the caller already set still wins.
  • A blank or whitespace-only value counts as unset, the same as the scanner's own .trim().
  • A plain ||= would keep " ", and the scanner would then fall back to HOME (the red run below shows this).
• tests/unit/plugins-fs-safety.test.ts: pins OMNIROUTE_PLUGINS_DIR under the file's own temp DATA_DIR before the manager singleton resolves its install root.
  • Before this change, the between-test cleanup re-derived the root from HOME.
  • The three staging-residue assertions inspected $TEST_DATA_DIR/plugins, a directory the manager never wrote to, so their existsSync guard skipped them every time. They now inspect the real install root.
• tests/unit/isolate-datadir.test.ts: adds child-process cases, in the same pattern as the existing DATA_DIR cases.
  • getDefaultPluginDir() resolves to $DATA_DIR/plugins when the value is unset, blank, or whitespace-only.
  • An explicit value is kept.

The resulting layout matches production, where DATA_DIR defaults to ~/.omniroute and the plugin dir to ~/.omniroute/plugins.

Validation (TDD)

Every run used the package.json import chain:

node --import tsx/esm --import ./open-sse/utils/setupPolyfill.ts --import ./tests/_setup/isolateDataDir.ts --test --test-force-exit <files>

Setup state	tests/unit/isolate-datadir.test.ts
Unmodified	3 pass, 1 fail: the resolver returned ~/.omniroute/plugins in place of $DATA_DIR/plugins
Plain ||=	3 pass, 1 fail: the " " case fell back to ~/.omniroute/plugins
This PR	4/4 pass

• plugins-fs-safety and plugins-scanner: 30/30 pass.
• tests/integration/plugins-lifecycle.test.ts: 17/17 pass.
• One batch run with APP_LOG_LEVEL=info: 507/507 pass.
  • It covered the 8 other plugin-manager suites, the 12 unit files that set OMNIROUTE_PLUGINS_DIR themselves, and all 28 harness-based unit files.
  • Every scanner.dir_resolved line in that run points at a temp directory or at one of plugins-scanner's fixed fixture values. None points at the real home plugin dir.
• eslint with config/quality/eslint-suppressions.json, and prettier --check: clean on the three changed files.