docs(changeset): add changeset for the Run-menu custom-model picker PR

Covers #430's full scope so far: the picker itself, the model-selection
dialog, periodic re-discovery, and the session-busy/toast/CLAUDE_CONFIG_DIR/
context-length/llama-swap-conflict fixes found through live validation
against a real llama-swap server.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RqZeHrRS6DYcGcGX2p9EwG

.changeset/run-menu-custom-model-picker.md

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+**Custom model endpoints: Run-menu picker, and hardening from real llama-swap validation** (#430, follow-up to #393's HTTP-API-only cut). With **Custom model endpoints** on (App Settings → Models) and at least one saved endpoint carrying a discovered model, the Run dropdown grows a **Custom Endpoints** section generated live off the CLI registry's own `capabilities.customModelInjection` — one entry per (harness that can redirect to a custom endpoint, saved endpoint). Picking one launches that harness and applies the endpoint to it; with two or more discovered models a small, scrollable dialog asks which one first, the endpoint's `defaultModelId` marked but never auto-chosen. Endpoints also now re-discover themselves automatically every 5 minutes in the background, one unreachable endpoint never blocking the others.
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+Everything below was found and fixed against a **real llama-swap server**, not just unit tests:
+
+- **Session-busy false refusal.** A freshly launched CLI reports itself `busy` for its own startup (spinner, workspace-trust check) well before the apply call would reach it, and the apply route correctly refuses to restart a session mid-turn — indistinguishable from a fresh boot. The picker now waits for the new session to go idle (bounded at 20s, never an error on timeout) before applying.
+- **Errors and confirmations you can actually read.** Toasts now default to sticky with a close button (errors always were meant to stay, but a fixed 3s timer silently hid them); a failed apply's real server-side reason (not a generic message) reaches the toast.
+- **"Both claude.ai and ANTHROPIC_API_KEY set" warning.** A custom-model Claude session now runs with an isolated `CLAUDE_CONFIG_DIR` (empty, no real credentials in it) so the injected API key never coexists with a stored OAuth login — `projects` is symlinked back to the real config dir so the response viewer/subagent windows/Read My Mind keep working. That isolated, otherwise-empty directory has none of a real profile's prior "Detected a custom API key — use it?" approvals either, which would otherwise re-ask on *every* launch with nobody at a TTY to answer (and silently refuse the key on its own default); the apply step now pre-seeds that exact approval field the same way answering the prompt once by hand would.
+- **Context-window overflow.** Claude Code assumes a large default context window for a model id it doesn't recognize and never compacts, so a real local model's much smaller context silently overflowed (confirmed live: a stock ~33.7K-token system prompt against a 16384-token model). Discovery now also learns each model's real context length from llama.cpp/llama-swap's `GET /props?model=`, but **only** for a model llama-swap's own `/v1/models` response already reports loaded — never an unloaded one, since asking about one risks triggering an actual, slow, GPU-swapping load as a side effect of read-only discovery — and applies it as `CLAUDE_CODE_MAX_CONTEXT_TOKENS`.
+- **The real root cause of "it still says opus, not my model."** llama.cpp runs exactly one model at a time; llama-swap unloads and reloads it on demand, which can take anywhere from a few seconds to well over a minute — long enough that a session mid-swap is indistinguishable from one that never left the native backend. Applying a selection now checks llama-swap's own `GET /running` first (feature-detected; a plain llama.cpp/OpenAI-compatible server has no such endpoint and is never checked); if switching would unload a model **another live session is actively using**, the apply is refused with a warning naming that session instead of silently switching, and a confirmation retry proceeds anyway. Either way, a sticky "loading model…" toast now covers the actual swap window until llama-swap reports the target model ready, so a prompt sent mid-swap reads as "loading," never as silence or an answer from whatever was loaded a moment before.
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+Remote (SSH) and Docker sessions are refused for now (400) — their restart reattaches the durable remote/in-container tmux rather than relaunching the agent.