fix(security): push-endpoint SSRF guard + tmux name validation; document tail-file roots

- M7 (SSRF): add isSafePushEndpoint (https-only; reject internal/loopback/link-local/metadata IPs incl. IPv4-mapped); enforce in PushSubscribeSchema and re-check before webpush.sendNotification. + unit test.
- M1 (command injection): validate tmux session names with isValidMuxName in sessionExists, killSession, and reconcileSessions before they reach a shell call site.
- M5: keep the intentional /var/log + ~/logs log-tail roots (a tested feature) and document the wider read scope in docs/security-architecture.md section 5 instead of dropping it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/utils/index.ts

@@ -22,6 +22,7 @@ export {
   execPattern,
 } from './regex-patterns.js';
 export { MAX_SESSION_TOKENS } from './token-validation.js';
+export { isSafePushEndpoint } from './push-endpoint-validation.js';
 export { stringSimilarity, fuzzyPhraseMatch, todoContentHash } from './string-similarity.js';
 export { assertNever } from './type-safety.js';
 export { wrapWithNice } from './nice-wrapper.js';