[v0.3] Add incremental search

[owainlewis]

Goal: Add Emacs-shaped incremental search for fast navigation inside the current buffer.

Context: Markdown-heavy work needs quick search before Cortex feels usable for real notes and docs.
The PRD reserves C-s for forward isearch and C-r for reverse isearch.
This should build on the minibuffer foundation if that has landed.

Proposed approach:

• Add search state that owns query text, direction, active match, and original point.
• Bind C-s to start forward search and repeat forward while search is active.
• Bind C-r to start reverse search and repeat backward while search is active.
• Update the active match as the query changes.
• Render the search prompt and a visible active match with restrained styling.
• Escape cancels and restores the original point.
• Enter accepts the current match and exits search.

Acceptance criteria:

• C-s starts forward incremental search.
• Typing updates the query without editing the buffer.
• Matching moves point to the current match as the query changes.
• Repeating C-s moves to the next match.
• C-r searches backward or repeats backward while search is active.
• Escape cancels search and restores the original point.
• Enter accepts the current match.
• No-match state is visible and does not move point unexpectedly.
• Existing editing, save, quit, command line/minibuffer, picker, and highlighting behavior still works.

Verification:

• Run cargo test.
• Manually search forward and backward in a Markdown file.
• Manually test no-match, cancel, accept, repeated search, save, and terminal cleanup.

Out of scope:

• Regex search.
• Replace.
• Project-wide search.
• Search history.
• Multi-buffer search.