transcript: search / filter over the scrollback

[foundev]

There is no way to search the transcript — finding an earlier answer, a file path, or a failed command means scrolling and eyeballing. Already listed in PLANS.md under M3 ("Search or filter over the transcript"); this issue tracks it concretely.

Sketch:

• / (or Ctrl-F) opens a search prompt in fullscreen and in the inline full-transcript reader; n/N jump between matches; matches highlighted.
• Search over the logical entry text (Entry in src/app.rs), then map hits to rendered rows, so wrapping doesn't split matches.
• A focus/selection model over entries (see transcript: per-tool-call expand/collapse instead of the global Ctrl-T toggle #342) would give search-hit navigation for free, and opens the door to per-entry copy and timestamps-on-demand.

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