feat(daemon): content-protocol RPC router and read methods

[tlgimenes]

Part of the v8 Studio stack, split from #7728, #7770 and #7766 (closed). Base: #7796. The top of the stack, #7843, has the same code as those three PRs combined.

The content-protocol JSON-RPC router and its read side: envelope and batch handling, request body limits, and the three read methods describe, schema.get and blocks.list with their parameter validation. Also adds secrets_test.go (the previous PR's secret guard plus describe's servablePublicKey). blocks.apply and the write paths come in the next PR.

What to look at

Envelope/batch handling and body limits in rpc.go/handler.go; ifNoneMatch short-circuits in schema.get/blocks.list; describe never broadcasts anything but a single PUBLIC KEY PEM block. An unknown method (including blocks.apply here) answers method-not-found.

Verified: go vet ./... && go test -race ./... in packages/sandbox/daemon-go pass on this branch.

Stack (Go daemon track 5/9, plan step 16a): previous #7815 · next #7817

🤖 Generated with Claude Code

https://claude.ai/code/session_01WNwbSEePYNcY5YCgqZURig

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Summary by cubic

Adds the content-protocol JSON-RPC router and its read side to the Go daemon: envelope and batch handling, request body limits, and the three read methods describe, schema.get, and blocks.list with parameter validation. blocks.apply and the write paths land in the next PR.

• ifNoneMatch short-circuits schema.get and blocks.list with a not-modified response.
• describe never broadcasts anything but a single PUBLIC KEY PEM block.
• Unknown methods, including blocks.apply here, answer method-not-found.
• Adds secrets_test.go covering the secret guard and describe's servablePublicKey.

^{Written for commit 807184c. Summary will update on new commits.}