fix(tui): dispose stale session UI renderers

[roboomp]

Repro

Starting a replacement chat after a prior session can leave transient TUI components detached but alive. I reproduced the render loop with bun .wt/repro-4686.js: after a status Loader was removed with Container.clear(), it still called requestComponentRender() (before=3, after=5) and exited with code 1. I also checked the reporter's resize follow-up with bun test packages/tui/test/resize-viewport-defer.test.ts packages/tui/test/issue-2088-repro.test.ts packages/tui/test/issue-2095-repro.test.ts, which kept the existing resize renderer regressions green.

Cause

Container.clear() in packages/tui/src/tui.ts only detaches children; it does not call child dispose() hooks. New-session and session-switch UI paths in packages/coding-agent/src/modes/controllers/*, interactive-mode.ts, and ui-helpers.ts used clear() for destructive transcript/status replacement, so active Loader/transcript components could keep timers alive and continue requesting TUI renders after the new chat or resize redraw started.

Fix

• Added Container.disposeChildren() in packages/tui/src/tui.ts for the common destructive-clear contract: dispose every child, then detach it.
• Routed new-session, handoff, extension/session-switch, transient status, and pending-message replacement paths through clearTransientSessionUi(), resetTranscript(), renderInitialMessages(), or disposeChildren() instead of non-disposing clears.
• Removed redundant pre-rebuild transcript clears so renderInitialMessages() can dispose the old transcript blocks itself.
• Added TUI regression coverage for disposeChildren() and for detached loader repaint timers stopping when a container is destructively cleared.

Verification

• bun test packages/tui/test/container-dispose.test.ts packages/tui/test/loader.test.ts → 11 pass, 0 fail.
• bun --cwd=packages/tui run check → passed.
• bun --cwd=packages/coding-agent run check:types → passed.
• bun --cwd=packages/coding-agent run check → passed.
• bun test packages/tui/test/resize-viewport-defer.test.ts packages/tui/test/issue-2088-repro.test.ts packages/tui/test/issue-2095-repro.test.ts → 26 pass, 0 fail.
• bun run fix → passed before commit.

Fixes #4686

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[roboomp]

No code change requested by this notice. PR #4693 remains ready for maintainer review; Codex review quota needs repo-admin action outside this branch.