feat: strengthen edge reporter module

[Gavriel-lab]

Summary

Implements the Edge Reporter Module bounty scope from the Clawland bounty board and closes #24.

This PR strengthens pkg/edge/reporter.go so PicClaw can act as a Fleet/MoltClaw reporting edge node with:

• startup registration to /fleet/register
• immediate and periodic heartbeats to /fleet/heartbeat
• status snapshots to /fleet/status
• event-driven reports to /fleet/events
• bearer-token propagation for fleet auth
• safe default heartbeat intervals when config is missing/zero
• idempotent Stop() to avoid double-close panics during shutdown
• endpoint joining that handles trailing slashes cleanly

I also added focused httptest coverage for registration, heartbeat, status, events, auth headers, HTTP error handling, default interval fallback, and idempotent stop behavior.

Validation

$ gofmt -w pkg/edge/reporter.go pkg/edge/reporter_test.go
$ go test ./pkg/edge
ok  	github.com/sipeed/picoclaw/pkg/edge	0.975s
$ git diff --check
# passed

I also ran go test ./.... The new edge package passed, but the full suite currently fails in the 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	0.901s

This PR only changes README.md, pkg/edge/reporter.go, and pkg/edge/reporter_test.go.

[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.