refactor(paths): consolidate ~/.socket dir constants, add _state, drop dead helpers

[John-David Dalton (jdalton)]

What

Reshape the ~/.socket/ directory constants + helpers in paths/socket.ts and constants/socket.ts.

• Add _state (~/.socket/_state) — version-less persistent app state: the home for daemon sockets, locks, OAuth refresh, durable caches that survive version bumps (mirrors pnpm state-dir / XDG_STATE_HOME). New getSocketStateDir / getSocketAppStateDir / getSocketAppRuntimeDir (the _state/<app>/run/ dir for a daemon's socket + concurrency.lock + pid) + RUN_DIR + getSocketStateDirEnv.
• Consolidate the infra-dir names into one SOCKET_DIR map holding full _-prefixed names (cacache/dlx/state/wheelhouse).
• Rename SOCKET_APP_PREFIX -> SOCKET_DIR_PREFIX — it prefixes dirs (including infra dirs like _cacache), not apps.
• Remove dead helpers with zero consumers: getSocketCliDir, getSocketRegistryDir, getSocketRegistryGithubCacheDir, plus orphaned SOCKET_CLI_APP_NAME / SOCKET_FIREWALL_APP_NAME / SOCKET_REGISTRY_APP_NAME.
• Collapse duplicated JSDoc blocks + refresh the stale @file layout comment.

Why

Foundation for the proteus credential-broker daemon (needs a stable, version-less _state/<app>/run/ home for its Unix socket + lock). Also removes accumulated dead code + a self-contradicting *_APP_NAME naming (the constants describe dirs, not apps).

Verification

• typecheck — 0 errors
• lint --all — 0 / 0
• 1761 tests pass (env + constants + paths + external-tools), 0 failures

getSocketAppCacheDir/getSocketAppCacheTtlDir kept (bundled into socket-cli dist); getSocketWheelhouseDir kept (used by janus).