mecatui: bound /worktrees picker #2172

Scope

Migrate the filterable /worktrees picker in cmd/mecatui/ui/worktrees.go to a height-bounded selectable list. renderWorktreesPanel currently ignores its offered height and renders every filtered worktree, allowing a long list to extend beyond the available conversation region. Keep the post-selection confirmation card out of scope.

Required outcome

• Budget title, explanatory text, filter, and footer before sizing the rows. Use bounded.List with stable worktree identities (not filtered indices), and presentListRow for standard cursor presentation; keep list rows and overflow within the offered width/height, including a compact fallback.
• Keep cursor and visible anchor stable on filtering/refresh when the same worktree remains; reveal the selected row and bound navigation. Preserve safe label/branch rendering, the filter, Enter confirmation, and the existing session handoff.
• Preserve current key semantics unless a new affordance is explicitly justified and tested.

Verification

Scenario-first tests: many worktrees at short heights, a filtered/updated inventory, resize, selection visibility and correct confirmation identity, safe untrusted labels, empty/error/loading states. Run focused UI and bounded tests.

Relates to #1742 and #1589.