Remove dead code: unused exports

[platypii]

What

Removes seven exported functions that have zero references anywhere in the repository (verified across .js, .json, .ts, .md, excluding node_modules and llp/), along with imports and a comment orphaned by the deletions.

Symbol	Location	Why dead
pluginInstallRoot	src/core/plugin_install/paths.js	No callers; pluginInstallDir covers real use
statusLogDir	src/core/daemon/status.js	No callers
platformIsSupported re-export	src/core/daemon/status.js	Existed only "for symmetry" with statusLogDir; nobody imports it from here
stopServiceDaemon	src/core/daemon/install.js	No callers (start/restart/status are used, stop never was)
isAttached	codex plugin settings.js	No callers
clearAiGatewayRuntime	ai-gateway runtime.js	Teardown stub never wired into lifecycle
clearGraphRuntime	context-graph runtime.js	Teardown stub never wired into lifecycle
clearGascityRuntime	gascity runtime.js	Teardown stub; the comment claiming tests use it was stale

Also removed now-orphaned imports (defaultLogDir, platformIsSupported in status.js; isManagedAttached in codex settings.js) and fixed the stale gascity comment.

Checks

• npm test — 1485 pass, 0 fail
• npm run lint — clean
• npm run typecheck — clean

First of three planned cleanup PRs (dead code, then em dashes, then type-import specifiers).

[platypii]

Code review (high effort) — clean

Ran a recall-biased review focused on the angles that matter for a deletion-only diff: removed-behavior, cross-file references, and orphaned imports.

Result: no findings. Verified grep-backed:

• All 7 deleted symbols (pluginInstallRoot, statusLogDir, platformIsSupported re-export, stopServiceDaemon, isAttached, clearAiGatewayRuntime, clearGraphRuntime, clearGascityRuntime) have zero remaining references anywhere in the repo (incl. dynamic imports, manifests, registries, tests).
• install.js: every remaining import (platformIsSupported, DaemonInstallError, getLogger, withSpan, Attr, defaultLabelFor, macos, linux) is still used by other functions after removing stopServiceDaemon — no orphans.
• status.js: defaultLogDir and platformIsSupported appear nowhere else in the file.
• codex settings.js: isManagedAttached is unused there (still defined/used in toml-config.js).

Checks: npm test (1485 pass), npm run lint, npm run typecheck all green. Merging.