feat: add Signal provider support

docs/signal.md

@@ -0,0 +1,46 @@
+# Signal (signal-cli)
+
+Status: external CLI integration only. No libsignal embedding.
+
+## Why
+- Signal OSS stack is GPL/AGPL; not compatible with Clawdis MIT if bundled.
+- signal-cli is unofficial; must stay up to date (Signal server churn).
+
+## Model
+- Run `signal-cli` as separate process (user-installed).
+- Prefer `daemon --http=127.0.0.1:PORT` for JSON-RPC + SSE.
+- Alternative: `jsonRpc` mode over stdin/stdout.
+
+## Endpoints (daemon --http)
+- `POST /api/v1/rpc` JSON-RPC request (single or batch).
+- `GET /api/v1/events` SSE stream of `receive` notifications.
+- `GET /api/v1/check` health probe (200 = up).
+
+## Multi-account
+- Start daemon without `-a`.
+- Include `params.account` (E164) on JSON-RPC calls.
+- SSE `?account=+E164` filters events; no param = all accounts.
+
+## Minimal RPC surface
+- `send` (recipient/groupId/username, message, attachments).
+- `listGroups` (map group IDs).
+- `subscribeReceive` / `unsubscribeReceive` (if manual receive).
+- `startLink` / `finishLink` (optional device link flow).
+
+## Process plan (Clawdis adapter)
+1) Detect `signal-cli` binary; refuse if missing.
+2) Launch daemon (HTTP preferred), store PID.
+3) Poll `/api/v1/check` until ready.
+4) Open SSE stream; parse `event: receive`.
+5) Translate receive payload into Clawdis surface model.
+6) On SSE disconnect, backoff + reconnect.
+
+## Storage
+- signal-cli data lives in `$XDG_DATA_HOME/signal-cli/data` or
+  `$HOME/.local/share/signal-cli/data`.
+
+## References (local)
+- `~/Projects/oss/signal-cli/README.md`
+- `~/Projects/oss/signal-cli/man/signal-cli-jsonrpc.5.adoc`
+- `~/Projects/oss/signal-cli/src/main/java/org/asamk/signal/http/HttpServerHandler.java`
+- `~/Projects/oss/signal-cli/src/main/java/org/asamk/signal/jsonrpc/SignalJsonRpcDispatcherHandler.java`