feat: per-thread session isolation via --session-id / --resume

[bchoor]

Problem

All messages across every channel, topic, and thread route to the same agent and share a single Claude session via -c (continue last). A Telegram topic and a Discord thread hitting the same agent bleed context into each other, and there's no way to have isolated conversations per thread without defining a separate agent per context — which doesn't scale.

Related: #125 #81 #72

Proposed Solution

Use --session-id (Claude) and resume <session-id> (Codex) to track multiple independent sessions per agent, keyed by (agent_id, thread_id) stored in the existing SQLite DB.

Key insight: Claude's --session-id <uuid> lets you pre-assign a UUID before the call — so you own the session ID from the start, no stdout parsing needed. -r <uuid> resumes it on subsequent messages.

New table:

CREATE TABLE thread_sessions (
    agent_id   TEXT NOT NULL,
    thread_id  TEXT NOT NULL,  -- e.g. "telegram:652", "discord:thread:abc"
    session_id TEXT NOT NULL,  -- UUID
    provider   TEXT NOT NULL,
    created_at INTEGER NOT NULL,
    updated_at INTEGER NOT NULL,
    PRIMARY KEY (agent_id, thread_id)
);

Invocation changes:

• First message in a thread → generate UUID, claude --session-id <uuid> -p "msg", persist it
• Subsequent messages → claude -r <uuid> -p "msg"
• Codex → codex exec resume <uuid> for existing threads, plain codex exec for new (parse session ID from JSONL output on first run)
• Reset → delete the row, next message starts fresh
• Fork → claude -r <uuid> --fork-session -p "msg" (free bonus)

Result

telegram topic 652  →  session UUID-A  (isolated)
telegram topic 29   →  session UUID-B  (isolated)
discord thread xyz  →  session UUID-C  (isolated)

Same agent folder. No config changes needed. Survives restarts. New threads get fresh sessions automatically.

Scope

~200 lines across 4 files:

• src/lib/db.ts — new table + getThreadSession / saveThreadSession
• src/lib/invoke.ts — replace -c with --session-id / -r <id> logic
• src/queue-processor.ts — pass threadId into invokeAgent
• src/channels/telegram-client.ts + discord-client.ts — set threadId from message_thread_id / channelId

Note: OpenCode has no --session-id equivalent yet — falls back to current -c behavior for that provider.