Agent runtime: majordomo in-process, Ollama Cloud config, chat endpoint (#56) (#70)

Co-authored-by: Steve Dudenhoeffer <[email protected]>

internal/store/migrations/0009_agent_chat.sql

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+-- Agent conversations (#56): multi-turn chat with the garden assistant.
+--
+-- "Now do the same to the north bed" needs the previous exchange, and history
+-- held only by the client would be lost on a refresh — which is exactly when
+-- someone reloads to see whether the agent's change actually landed.
+--
+-- One conversation per (user, garden). Two people editing a shared garden get
+-- separate threads: a conversation is a dialogue with one person, and merging
+-- them would put words in someone's mouth.
+CREATE TABLE agent_conversations (
+    id         INTEGER PRIMARY KEY,
+    garden_id  INTEGER NOT NULL REFERENCES gardens (id) ON DELETE CASCADE,
+    user_id    INTEGER NOT NULL REFERENCES users (id)   ON DELETE CASCADE,
+    created_at TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    updated_at TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
+    UNIQUE (garden_id, user_id)
+);
+
+-- Only the user/assistant TEXT of each turn is kept, not the full model
+-- transcript with its tool calls. Continuity needs what was said and what came
+-- back; replaying a stored tool call would be replaying a decision made against
+-- a garden that has since moved on. It also keeps the schema free of majordomo's
+-- message shape, which is a dependency's business, not the database's.
+CREATE TABLE agent_messages (
+    conversation_id INTEGER NOT NULL REFERENCES agent_conversations (id) ON DELETE CASCADE,
+    id              INTEGER PRIMARY KEY,
+    role            TEXT    NOT NULL CHECK (role IN ('user', 'assistant')),
+    body            TEXT    NOT NULL,
+    -- The change set this turn produced, so the panel can offer Undo on the
+    -- message that caused it. SET NULL rather than CASCADE: losing the history
+    -- entry must not delete what was said about it.
+    change_set_id   INTEGER REFERENCES change_sets (id) ON DELETE SET NULL,
+    created_at      TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
+);
+
+CREATE INDEX idx_agent_messages_conversation ON agent_messages (conversation_id, id);