Open palette search results in splits

[brsbl]

Human comments

What was wrong

Thread search results could only replace the current view; users could not open a result beside their existing thread.

What changed

• Open selected thread results in splits with Command/Control-Enter.
• Preserve message-match anchors when opening a split.
• Reveal the split hint only while holding the modifier with an eligible selection and focused input.

How you verified

• Previously reviewed behavior; no additional code review.
• Head 224986c1a: remote CI failed downloading Linux FUSE; failed job rerun requested.
• Chrome for Testing 153, 1440×900: header button absent; modifier-held hint appears and clears; Command-Enter opens two panes.
• Updated regression coverage for button absence, keyboard opening, and conditional hints.
• Parent/head comparison evidence remains outstanding; this PR remains draft.

Header refinement: previous candidate → current head (2× desktop captures)

These show the header-button removal from e255acf0a to 224986c1a, not the PR-parent comparison. Live relative timestamps and background loading differ.

Before — e255acf	After — 224986c

BB-Thread-ID: thr_82jtfaq8eq

AGENT GENERATED