refactor(coding-agent): delete unreachable empty-selector auto-cancel timers

[snimu]

What was wrong

Both TUI selectors (fork-message and session-tree) carried a 100ms auto-cancel timer for the empty-collection case — but their only construction sites already return early with a user-facing notice when the collection is empty. Unreachable defensive timers, plus the stale-callback hazard they imply (audit: timeouts.md finding 8).

The fix

Pure deletion of both blocks (+1/−8). No replacement assertions — the callers own the precondition.

How it's verified

Reviewer independently enumerated all construction sites (production + tests) confirming the empty case cannot reach the components, and that the deleted blocks held no listener registrations — nothing leaks. tsgo/biome clean, tree-selector 18/18, full CI-style suite failing set matches stack base. Two-model implement/review loop, approved first pass.

Stacked on #1744 (test the whole stack at the leaf; merge base-first). This completes the deletion-only wave of the audit stack.

Note: intentionally no Linear ticket for this cleanup stack, so that check stays red.

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Note

Low Risk
Dead-code removal with no change to reachable paths; callers already enforce non-empty collections before opening selectors.

Overview
Removes 100ms setTimeout auto-onCancel logic from TreeSelectorComponent and UserMessageSelectorComponent constructors when given an empty tree or message list.

Those branches were unreachable in production: showTreeSelector and showUserMessageSelector in interactive mode already bail out with a status message before constructing the components. This is deletion-only cleanup from the timeouts audit (stale callback hazard), with no new guards added inside the components.

A changelog note documents the removal of delayed cancellation from empty interactive selectors.

^{Reviewed by Cursor Bugbot for commit 2033172. Bugbot is set up for automated code reviews on this repo. Configure here.}

Linear ticket: ENG-5663
(ticket linked above)

Note

Remove unreachable empty-selector auto-cancel timers in TreeSelectorComponent and UserMessageSelectorComponent

• Removes the setTimeout-based auto-cancellation from the constructors of tree-selector.ts and user-message-selector.ts
• Both selectors previously auto-invoked onCancel after 100ms when given an empty tree or messages array; this logic was unreachable and is now deleted
• Behavioral Change: empty selectors no longer auto-cancel; they remain active until explicitly canceled by the user or caller

^{Macroscope summarized 2033172.}

[snimu]

Superseded by #1828: GitHub's stack lock prevents retargeting stacked PRs, so this PR was recreated as a plain PR against main (same branch, same content, all review threads here are resolved). Please review/merge #1828 instead.