Secured Signal API Implementation

[pr0ton11]

Summary

Adds Signal as a new communication channel for picobot, using the secured-signal-api proxy in front of signal-cli-rest-api.

Architecture decisions

• WebSocket for inbound messages — connects to ws://<host>/v1/receive/<number> with automatic reconnection (exponential backoff, 1s → 30s). HTTP long-polling was avoided because it destructively consumes messages from the queue, which would break other consumers of the same Signal API instance.
• HTTP POST for outbound messages — sends via POST /v2/send with Bearer token authentication through the secured-signal-api proxy.
• No new dependencies — gorilla/websocket was already an indirect dependency (via discordgo); it has been promoted to direct.

What's included

Commit	Description
deb48b0	SignalConfig struct and Signal field in config schema
49a298d	signal.go — full channel implementation (~300 lines): WS inbound, HTTP outbound, echo suppression, allowFrom filtering, group support, message splitting
f302841	signal_test.go — 8 unit tests covering: empty params validation, WS URL building, round-trip inbound/outbound, group messages, allowFrom filtering, non-data message filtering, echo suppression, Bearer auth verification
5cb9675	Gateway wiring + interactive channels login menu with Signal option (5)
8d1ad77	Docker env var support: SIGNAL_API_URL, SIGNAL_API_TOKEN, SIGNAL_NUMBER, SIGNAL_ALLOW_FROM

Design notes

• Follows the existing channel convention exactly: StartSignal() accepts (ctx, hub, ...params), calls hub.Subscribe("signal"), launches goroutines for inbound/outbound.
• Uses the signalSender interface pattern (matching discordSender/slackPoster) for testability — all tests use mock HTTP servers, no real Signal API needed.
• Reuses the existing splitMessage() helper from discord.go for message chunking.
• The + in phone numbers is percent-encoded as %2B in the WebSocket URL path, as expected by signal-cli-rest-api (Go's url.PathEscape considers + valid in paths and leaves it literal).

Verification

• go build ./... — clean
• go vet ./... — clean
• go test ./... — all pass (8 new Signal tests + all existing tests)

[pr0ton11]

@louisho5 may I bother you for a review? Thank you

[louisho5]

Thanks @pr0ton11 :) It is a really cool suggestion. I will explore it.

[pr0ton11]

I'll fix the linter issues.

[pr0ton11]

Done. The linter issues were fixed. @louisho5 Please give it a review.