feat: port signal channel adapter from TS to Rust

Replaces src/channels/signal.ts (107 LOC) with workers/channel-signal/
(~310 LOC Rust + tests). Inbound webhook + outbound REST bridge to
signal-cli-rest-api (no Node-only deps required — pure HTTP).

Function IDs preserved:
  - channel::signal::webhook

HTTP trigger: POST /webhook/signal
Auth env: SIGNAL_API_URL, SIGNAL_PHONE (resolved via vault::get fallback
to env var). Group routing via envelope.dataMessage.groupInfo.groupId.

Outbound respects 10s send timeout (mirrors AbortController behavior
from TS). Empty agent reply still returns 500 to surface upstream
failures, matching TS port.

Continues #41 phase 3 channel migration.

Author: rohitg00

Cargo.lock

@@ -10,6 +10,7 @@ members = [
     "workers/approval-tiers",
     "workers/bridge",
     "workers/channel-email",
+    "workers/channel-signal",
     "workers/channel-slack",
     "workers/channel-teams",
     "workers/channel-webex",

Cargo.toml

@@ -0,0 +1,19 @@
+[package]
+name = "agentos-channel-signal"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[[bin]]
+name = "agentos-channel-signal"
+path = "src/main.rs"
+
+[dependencies]
+iii-sdk.workspace = true
+tokio.workspace = true
+serde.workspace = true
+serde_json.workspace = true
+tracing.workspace = true
+tracing-subscriber.workspace = true
+anyhow.workspace = true
+reqwest = { version = "0.12", features = ["json", "rustls-tls"], default-features = false }

src/__tests__/channels-signal.test.ts

@@ -0,0 +1,7 @@
+iii: v1
+name: channel-signal
+language: rust
+deploy: binary
+manifest: Cargo.toml
+bin: agentos-channel-signal
+description: "Signal channel adapter — REST bridge to signal-cli-rest-api"