[FEATURE]: Search across my message history to quickly locate past conversations/content ("find what I told opencode before")

Problem

Over time I have hundreds of sessions across many projects. When I remember that "I told opencode something important a while ago" (a requirement, a decision, a constraint, a command I typed), there is currently no way to find it:

• The TUI/Desktop session list only shows sessions by title/project — no content search.
• Ctrl+F inside a session (#19143) only searches the currently open session — it cannot find that phrase I typed three weeks ago in another project.
• The only workaround is recalling the approximate session/title, then scrolling through it manually. For long-running threads this is extremely inefficient; for forgotten sessions it's often impossible.

This is especially painful for agent-driven work: users increasingly delegate multi-day tasks, and "what did I tell the agent earlier" becomes a recurring lookup need.

Feature request

Add a content search across the user's message history:

1. Scope: search my (user) messages — preferably all sessions, with per-project filtering (project:name, date range).
2. Trigger: a search entry from the session list / project page (e.g. a search box or a /search command in the TUI); shortcut in Desktop.
3. Result view: list of matches with session title, project, timestamp, and the surrounding message snippet; clicking a result opens that session (optionally scrolled to the matched message — which is exactly what #19101-style in-session find would then support).
4. Constraints: search user messages by default; option to include assistant messages/tool calls; fuzzy match would beat SQL-like exact match for human recall.

Relationship to existing issue

• #19101 covers in-session Find (Cmd/Ctrl+F) — complementary, not overlapping: that's "find in the session I'm looking at", this is "find which session contains it" (session discovery by content). Implementing this makes #19101 more useful and vice versa.

Environment

• OpenCode Desktop 1.18.15 + CLI, Windows 11
• Many projects/sessions accumulated daily; data presumably already stored in local session store (opencode.*.dat), so a local index/search over it should be feasible without server-side changes.