Fix messages --sync behind live tail

[steipete]

Summary

• Make messages --sync fail fast with an omit---sync hint only when a live tail process owns the sync lock.
• Keep plain messages as a local read, and treat startup/stale/malformed/legacy lock metadata as an ordinary writer that preserves blocking behavior.
• Publish active tail ownership only after the Discord gateway opens; clean up owner-token metadata on cancellation/error.
• Update help/docs/changelog with thanks to @jeanmonet.

Closes #81

Proof

• go test -count=1 ./...
• go test -race -count=1 ./internal/cli ./internal/syncer ./internal/discord
• go vet ./... && go build ./...
• GOOS=windows go test -c -o /tmp/.../cli.test.exe ./internal/cli
• Process proof: active tail lock exits 2 with tail already owns live sync; omit --sync while tail is running.
• Process proof: stale tail-looking metadata without owner token blocks, with no tail-specific hint.
• Process proof: tail-starting metadata blocks, with no tail-specific hint.
• $autoreview: clean, no accepted/actionable findings.