Search: in-channel FTS (/) and workspace search modal (ctrl+f)

[gammons]

Summary

• In-channel search (/): vim-style search over the current channel's cached history, backed by a new SQLite FTS5 index (messages_fts, external-content table with trigger sync + backfill; LIKE fallback if FTS5 is unavailable). Matches highlight in the message pane; n/N jump between them (wrapping), status line shows /query 3/17, Esc clears. Accent/case-insensitive word-prefix matching.
• Workspace search (ctrl+f): modal backed by Slack's search.messages with raw query pass-through (from:@user, in:#channel, before: modifiers work). Results render as structured blocks — #channel author date, time header with indented 2-line snippets, searched terms highlighted, mrkdwn entities flattened (mentions/links/usergroups), DM names resolved. Modal sizes to 70% of the terminal; Enter jumps to the message (thread replies open the thread panel); hits in non-member channels toast instead of dead-ending.
• Jump-to-message history loading (FetchAround): fetches a window around a target ts when it's outside the loaded buffer — used by search jumps and also upgrades the permalink-navigation path from Add 'o' keybinding to open links, with in-app Slack permalink navigation (#62) #71 (which previously toasted "older than loaded history").

Notable hardening along the way: ANSI/OSC-safe highlight renderer, generation-counter guards against stale async search results, anchor-guarded older-history prepends (fixes a latent buffer race), read-only user lookups on the search goroutine (avoids a concurrent map write), contiguous-window guarantee in GetHistoryAround.

Design doc: docs/superpowers/specs/2026-06-10-search-design.md
Plan: docs/superpowers/plans/2026-06-10-search.md

Test Plan

• go test ./... (48 packages), go vet ./..., -race on the new search paths
• Live verification against a real workspace: xoxc tokens work with search.messages; / highlights + n/N + off-buffer jumps; ctrl+f modifiers, cross-channel/thread jumps, non-member toast; permalink jump to old message
• FTS migration smoke on a large existing cache.db (backfill runs once on first start)

Known v1 limitations (deliberate)

• First 50 server results only ("showing K of N" footer)
• No "load newer" path back to present after jumping deep into history (re-select the channel)
• Live messages append to a historical window without a gap marker
• Reply text is excluded from in-channel / search (replies aren't displayable in the channel pane); workspace search covers them