fix: start draft sessions from default model/agent and honor OpenCode default_agent

A new draft session inherited the previous session's model/agent instead of
resetting to defaults, because opening a draft restored the directory snapshot
without re-applying the startup default cascade. When the prior session ran in
a worktree, defaults were resolved against the worktree directory's provider
list, which omits project/global-scoped providers, so the default agent's model
fell back to opencode/big-pickle.

Resolve the default agent/model via a shared cascade (settings default ->
OpenCode default_agent -> build -> first), resolve the model from the agent's
pinned model/variant or OpenCode's config model, and activate the project's
config (not the worktree's) when opening a draft.

CHANGELOG.md

@@ -26,6 +26,9 @@ All notable changes to this project will be documented in this file.
 - Notifications: notification streams now stay connected more reliably behind proxies (thanks to @kostazol).
 - Mobile: the empty Changes view keeps a close control visible (thanks to @lilyzhaun).
 - Security: file previews and downloads now reject paths outside the allowed workspace unless access has been granted.
+- Sessions: fixed a bug where a running session would briefly flicker as idle (in the sidebar, the send/stop button, and the status row) when the app is protected by a password.
+- Desktop: you can now open developer tools from the Help menu.
+- Sessions: new draft sessions now start from the default model and agent instead of inheriting the previous session's selection, and fall back to OpenCode's own `default_agent` (and its model) when no OpenChamber default is set.
 
 ## [1.12.4] - 2026-06-11