fix(cache): wait for packed manifest discovery

[cre4ture]

Summary

• use the configured backend timeout while packed manifests are discovered before the cache server becomes ready
• retain the short 15-second startup check for object mode
• document the shared deadline, test the mode-specific behavior, and rebuild the v0.3.1 artifacts

Why

A real IronMesh cache seed currently contains 172 PAC manifests. The first read-only restore correctly discovers them before listening, but v0.3.0's Action wrapper stopped waiting after 15 seconds. This change lets the wrapper wait for the same bounded timeout that the server already uses for discovery, preventing false startup failures and early cache misses.

Validation

• node --test action/*.test.js
• go test ./...
• go vet ./...
• staticcheck, govulncheck, and actionlint
• rebuilt v0.3.1 distributions; checksums verified and a second build was byte-identical