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/file-stream-manager.ts

@@ -395,8 +395,12 @@ export class FileStreamManager extends EventEmitter {
     // Normalize the working directory
     const normalizedWorkingDir = resolve(workingDir);
 
-    // Check if the resolved path is within the working directory
-    // or common log directories (/tmp intentionally excluded — world-writable)
+    // Allowed read roots for log tailing: the session working dir plus the
+    // INTENTIONAL log directories (/var/log, ~/logs). This is wider than the
+    // per-session boundary used by validateSessionFilePath — a deliberate,
+    // tested design choice for tailing system/app logs, documented as such in
+    // docs/security-architecture.md (security review M5). /tmp is excluded
+    // (world-writable).
     const allowedPaths = [normalizedWorkingDir, '/var/log', resolve(homedir(), 'logs')];
 
     const isAllowed = allowedPaths.some((allowed) => {