0.6 prerelease checklist: stabilize guest-agent API and SDKs

[h4x3rotab]

Objective

Ship dstack 0.6 with a stable, versioned guest-agent API and consistent Go, JavaScript/TypeScript, Python, and Rust SDKs. Existing 0.5.x clients must continue to work against a 0.6 agent.

Compatibility

• Keep the unversioned API fully compatible with 0.5.x, verified with v0.5.10 generated clients and SDKs. — ci(sdk): run released 0.5.x SDK suites against the current agent #1121 runs the released SDK suites against the current agent in CI. The pinned tags are v0.5.8 and v0.5.11, not v0.5.10: v0.5.10:sdk and v0.5.11:sdk are the same tree object, so running both bought nothing. Note the limit — old JSON clients ignore unknown keys, so this job catches breaking changes to the frozen surface but not additive ones; the shape freeze in frozen_surface is what covers additions.
• Keep GetQuote unchanged: the v0.5 response fields and semantics remain the complete contract. Cross-platform attestation is exposed through Attest, not GetQuote.
• Keep GetKey wire- and behavior-compatible. Publish a normative specification for path, purpose, supported algorithms, defaults, output encoding, and signature-chain verification. — spec in docs: give the frozen v0 guest API a written contract #1123 (docs/guest-api-v0.md), byte-level: HKDF salt, what enters the KDF vs only the claim, both chain-link preimages, all three signing modes, and the hazard that algorithm does not domain-separate.
• Make EmitEvent return 404 with informative error message (deprecated method)
• Unknown algorithms must be rejected.

Attestation

• Make Attest the single API for platform and GPU attestation

SDKs

• Release all changed SDKs as 0.6.0 and apply semantic versioning consistently.
• Expose the same TlsKeyOptions fields and defaults in every SDK. usage_server_auth defaults to true; remove the unused path option. — sdk: finish the 0.6 cleanups — v1 defaults, v1 naming, transport errors #1120
• Represent the TLS private key as PEM and provide an explicit toPkcs8Der() conversion. Remove ambiguous byte-conversion APIs. — resolved differently, deliberately. The ambiguous accessor is gone from v1 (sdk: finish the 0.6 cleanups — v1 defaults, v1 naming, transport errors #1120) and no toPkcs8Der() replaced it: v1 returns PEM and nothing else. The accessor existed to feed the key into the blockchain adapters, v1 has no chain-flavored surface, and DER is one standard-library call away for anyone who wants it. v0 keeps the accessor, because the adapters are v0-typed and that surface is frozen. Reopen this if a v1 caller turns up who needs DER from the SDK itself.
• Represent protobuf byte fields as bytes in Rust, not hex strings. — sdk: give v1 byte fields the type the proto declares #1124 (open), and in every SDK rather than Rust alone: eleven response fields plus report_data, with the decode_* helpers deleted. The JSON wire is unchanged.
• Make all SDKs report non-2xx responses consistently, including the server error and HTTP status. — sdk: finish the 0.6 cleanups — v1 defaults, v1 naming, transport errors #1120, pinned by tests in sdk: finish the 0.6 cleanups — v1 defaults, v1 naming, transport errors #1120 that assert the exact status an image without nvattest answers.
• Regenerate and review the protobuf, HTTP, and SDK reference documentation. — v1 and v0 both have written specs now (docs/guest-api-v1.md, docs/guest-api-v0.md) and sdk/curl/api.md was corrected, but the per-SDK reference docs have not been regenerated.

Versioned API (future)

• Introduce the long-term API under a versioned URL/service namespace such as /prpc/v1/... and protobuf package dstack.guest.v1. API selection must not depend on request headers. — guest-agent: freeze the v0 API at v0.5.11 and add the versioned v1 API #1116; selection is by URL path alone.
• Use *Request and *Response consistently for new messages. Keep the method name Attest.
• Give opaque JSON fields explicit *_json names in v1, or use typed messages where the schema is stable. — not done. InfoResponse still carries app_compose, vm_config and key_provider_info as bare string fields whose comments say they are JSON documents passed through unparsed. v1 is unreleased, so this is still cheap to change.

Signing (future)

• Provide Sign with an explicit key specification containing path, purpose, and algorithm. — v1 has no Sign; the frozen v0 Sign is now specified in docs/guest-api-v0.md.
• Add GetSigningKey to return the corresponding public key and certification chain without signing a message. — v1 GetKey returns public_key and signature_chain alongside the key, which covers the use case but not as a separate method.
• Provide signature verification as SDK functionality rather than an agent RPC. — refactor!: move signature verification out of the CVM and into the SDKs #1110. v0's Verify RPC is still served for 0.5.x clients and is documented as frozen.
• Specify the exact Ed25519 and secp256k1 message and prehashed modes. — done for the frozen v0 surface in docs: give the frozen v0 guest API a written contract #1123, including secp256k1_prehashed requiring the caller to supply the digest. v1 has no signing surface to specify.