feat: wire edge HTTP API to message bus

[Gavriel-lab]

Summary

• add the bounty-required Edge HTTP API routes: GET /api/health, GET /api/status, POST /api/command, and POST /api/message
• keep existing /healthz and /api/v1/* routes as compatibility aliases
• wire accepted commands/messages into the existing pkg/bus.MessageBus instead of only logging inbound requests
• pass the gateway message bus into the edge server startup path
• add focused pkg/edge handler tests for health/status, command publishing, message publishing, and invalid JSON
• document the new API routes in the README

Closes #22

Validation

• git diff --check passed locally
• I could not run go test ./pkg/edge in this Windows Codex environment because the Go toolchain is not installed (go / gofmt commands are unavailable). The PR includes focused tests for maintainers/CI to run.

/claim #22

[Gavriel-lab]

Validation update from my Windows workstation:

• Installed and verified Go go1.26.3 windows/amd64 from the official archive.
• Ran gofmt on the touched Go files and pushed the formatting cleanup in 6d7665e.
• Ran the focused package test for this PR:

$ go test ./pkg/edge
ok  	github.com/sipeed/picoclaw/pkg/edge	0.583s

I also checked the broader suite with go test ./.... The new edge package still passed, but the full suite currently fails in the pre-existing, untouched pkg/gene package:

--- FAIL: TestSelectGenes_SortedByScore (0.00s)
    selector_test.go:175: expected highest-scoring gene first, got low
FAIL	github.com/sipeed/picoclaw/pkg/gene	0.573s

This PR only changes README.md, cmd/picoclaw/main.go, pkg/edge/server.go, and pkg/edge/server_test.go; it does not touch pkg/gene.

[Gavriel-lab]

Follow-up on the full-suite validation note: I opened a separate minimal baseline fix for the existing pkg/gene selector ordering failure here:

#26

That PR keeps the fix isolated from this edge API work. Locally, after the gene selector tie-breaker fix, go test ./... passes on the base tree.