[codex] Fix claw-wall Discord rate limiting

[mostlydev]

Summary

• select exactly one claw-wall reader token per consumed Discord channel and inject an explicit wall poll interval
• add typed Discord 429 parsing plus channel-scoped and token-scoped cooldown tracking in claw-wall
• expand tests for wall selection, poll interval defaults, rate-limit parsing, cooldown behavior, and cooldown expiry

Root Cause

claw-wall was polling duplicate channel/token pairs and retried every non-200 response on the next tick, which could amplify Discord rate limits across multi-bot pods.

Validation

• go test ./cmd/claw-wall/... -count=1
• go test ./cmd/claw -run 'TestInjectConversationWall|TestPrepareClawAPI|TestValidateClawAPI' -count=1
• go test ./... -count=1
• go vet ./...

Closes #147.