Escalate agent Close to SIGKILL so it terminates in one action

Agent 'Close' (agent-close) sent a single SIGTERM via Session.Kill and
never escalated, so an agent that traps/ignores SIGTERM (e.g. opencode)
stayed in the running tab bar until the user closed it again. Add
Session.Terminate, which reuses terminateAndWait (SIGTERM, wait, then
SIGKILL) but preserves the session entry so the exited pane stays
readable, and route handleChildClose's agent path through it in a
goroutine to keep the UI responsive during the stop timeout.

Resolves the opencode double-close TODO item.

CHANGELOG.md

@@ -11,6 +11,9 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
   to remove a shared project scratchpad.
 
 ### Fixed
+- Closing an agent now escalates from SIGTERM to SIGKILL when needed,
+  so agents that ignore SIGTERM disappear from the running tab bar
+  after one Close action while keeping their exited pane readable.
 - Sidebar timer indicators now repaint as their visible countdown
   value changes, so labels progress from minutes to seconds without
   waiting for unrelated terminal output or focus changes.