Add web chat tab, 24h archive policy, and clear-chat

Setup page gets a copyable install command; /chat shows the live log,
posts as 'user', and has a two-click clear button. Messages older than
24h (or below the clear watermark) are archived out of all default
views and inboxes but never deleted; ?archived=1 retrieves them.

AGENTS.md

@@ -3,7 +3,7 @@
 ## Architecture
 
 - `src/server.ts`: Bun.serve HTTP handler (exported as `handle` for tests), routes, long-poll loop, and the server-rendered home page.
-- `src/db.ts`: bun:sqlite setup and all queries. DB path comes from `AGENTCHAT_DB` (default `data/agentchat.db`, `:memory:` in tests).
+- `src/db.ts`: bun:sqlite setup and all queries. DB path comes from `AGENTCHAT_DB` (default `data/agentchat.db`, `:memory:` in tests). "Archived" is derived at query time: older than 24h OR at/below `settings.archive_watermark` (which "clear chat" moves to MAX(id)); rows are never deleted.
 - `skill/SKILL.md` + `install.sh`: served assets. `{{BASE_URL}}` is replaced with the requesting host at serve time — keep the placeholder intact.
 - `docker/`: pinned-Bun Dockerfile, prod `compose.yml` (external `nginx-bridge` network, watchtower label), and `compose.local.yml` for local container runs.
 - `.gitea/workflows/build.yml`: quality gate (typecheck + test) then image push to `git.gbrown.org/gib/agentchat`.