fix(sessions): make the session search field recognizable

[liwen-aaa]

Problem

Fixes #553. In /sessions, the dim Filter: type to filter prompt blends into ordinary list text and does not clearly look editable.

Value

Makes session search easier to discover and visually separates the query from the results.

Approach

• Render ⌕ [ type to filter ] with accent delimiters while the list is focused, a muted placeholder, and normal text for an entered query.
• Share the same search field and horizontal separator across single-pane and preview layouts.
• Reserve space for the separator and cap visible list items to the available height.
• Truncate long queries while preserving the closing delimiter; also truncate the narrow-screen empty-result message.

Validation

• bun run check: passed (existing web bundle size warning).
• Sessions Node tests: 35/35 passed.
• Component render harness using the real picker: passed at 20, 40, 80, 120 and 160 columns, including filtering, long queries and no matches. This is component-level evidence, not a real terminal smoke test.
• bun run test: completed with exit 1. Parallel Node group: 1,561 tests, 1,548 passed, 3 failed, 10 skipped. Failures: file-mutation-display renderer ownership (undefined vs self), git-info process exit code (-1 vs 7), and workflows detached launch-card settlement timeout. Baseline reproduction has not been established.
• Vitest run separately with explicitly selected .spec.ts files: 220/220 passed.
• Windows serial background-terminal group run separately: reached the 90-second bound without a complete summary; several kill/process-tree cases failed. This group is not passing or fully verified.
• Real Pi TUI visual smoke: not yet verified. Default installed OpenPI is still npm version 0.5.0; it has not been switched to this checkout.
• git diff --check: passed.

Impact

User-visible: clearer session search and a separator consuming one additional row. Model-facing tools/context, runtime lifecycle, persisted configuration and session data: unchanged. Compatibility risk is limited to terminal layout and glyph rendering. Kept as a draft pending full validation and real terminal visual review.

[tt-a1i]

审查提交：bb3d27cd62c84fccc3353dd738f3d7d0cb59960d。

本轮未发现新的阻断问题。以下结论仅覆盖所列验证范围，不是合并批准。

验证范围

Complete one-file rendering diff inspected: widths bounded, two filter rows deducted from pane budget, text escaped by theme path; no persistence or execution changes.

限制与后续

Draft; no TUI visual acceptance, no new interaction tests. Height calculation also changes maxVisible behavior and should be visually checked.