v1.30.1 — pin claude to Opus 4.8 (1M)

Pins the built-in claude participant spec from the opus alias to the exact model ID claude-opus-4-8[1m] (Opus 4.8, 1M context). The alias resolved to 'latest opus' which mis-landed on an older Opus (~4.6) on some installs/accounts; parley run now always launches 4.8. CLI-verified the ID; acp_specs_test updated; build + go test ./... green.