[FEATURE]: Long-term persistence memory system (/teach, /recall, /learn, /memory)

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Describe the enhancement you want to request

Feature hasn't been suggested before

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Add a persistent long-term memory engine backed by SQLite (memory.db) to allow OpenCode to retain user conventions, architectural guidelines, and learned lessons across sessions.

Proposed Capabilities:

1. /teach <context>: Persist user instructions, architectural preferences, and rules.
2. /recall <query>: Retrieve relevant memories via SQLite FTS5 full-text search.
3. /learn [content]: Automatically extract or manually save lessons and insights discovered during coding sessions.
4. /memory [query]: Explore, search, filter, and inspect saved memories via CLI and TUI dialog.

Benefits:

• Eliminates repeating context, repository guidelines, and coding preferences across new sessions.
• Fast, local, zero-dependency persistence using SQLite WAL mode and FTS5 indexing.
• Accessible conversationally by the agent (tool call), via CLI commands (opencode memory ...), and via TUI slash commands and dialogs.