v0.51.12 — 3-PR batch (cron subprocess return + custom provider routing + session runtime invariants)

🐛 Fixed

#1746 by @Michaelyklam — Shorten cron profile lock for manual runs. Closes #1574. Returns from v0.51.11 deferral with the queue-drain blocker fixed (drain-then-join pattern + 200K-char regression test). Cron job body runs in a subprocess pinned to the selected profile context; parent stays responsive.
#1752 by @Michaelyklam — Route custom_providers[].models dict selections to custom:<name> (slice of #1240 source-of-truth umbrella). Reconciles still-relevant slice from stale conflicting #1311.
#1753 by @Michaelyklam — Guard session-owned runtime invariants (refs #1694). Background terminal events no longer tear down active-pane runtime when an unrelated session finishes; approval/clarify pollers stop by owner-session.

4622 → 4632 passing (+10 regression tests). 0 regressions. Stably green on first try.

All 3 PRs CI-green + rebased onto master with NO conflicts (disjoint files: api/config.py, static/messages.js, api/routes.py).
All JS files syntax-clean.
Browser API sanity (11/11 endpoints) — all pass.
Opus advisor: SHIP all 3, 0 MUST-FIX, 1 SHOULD-FIX filed as #1754 (fork→spawn migration in cron subprocess, architectural follow-up).

Full Changelog: v0.51.11...v0.51.12