#11107: fix(models): auto-allow selected model on sessions.patch

## Summary - When `sessions.patch` fails with `model not allowed: ...`, parse the disallowed model key. - Auto-add that model to `agents.defaults.models` in config. - Persist config via `writeConfigFile(...)`, reload config, and retry `applySessionsPatchToStore(...)` once. ## Why Users can select a valid model in UI/TUI but still hit `model not allowed` if allowlist is missing that key. This change unblocks the update flow without requiring a manual config edit. ## Scope - File: `src/gateway/server-methods/sessions.ts` - No behavior change for other error types. - No change if model already exists in allowlist. ## Validation - Manual: reproduced `model not allowed` on `sessions.patch`, then verified retry succeeds after auto-allowlist write. - Diff is intentionally minimal and isolated to `sessions.patch` path. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR changes the `sessions.patch` handler to detect `model not allowed: …` failures, persist the selected model into `agents.defaults.models` in the user config, reload config, and retry applying the patch once. The change is isolated to `src/gateway/server-methods/sessions.ts` and wraps the existing session store update/apply flow with a single retry after a config write when the failure is specifically a disallowed model. <h3>Confidence Score: 3/5</h3> - This PR is mostly safe, but the new auto-allow retry path may not trigger due to error-shape mismatch. - Changes are localized and gated behind a specific failure, but the implementation assumes `applied.error.message` contains the canonical `model not allowed: …` text; if the error is a string or a non-Error shape, the regex extraction fails and the intended fix won’t work. - src/gateway/server-methods/sessions.ts <!-- greptile_other_comments_section --> _{(3/5) Reply to the agent's comments like "Can you suggest a fix for this @greptileai?" or ask follow-up questions!} <!-- /greptile_comment -->