feat(provider): native AWS Bedrock (Converse API, hand-rolled SigV4)

[prashar32]

Implements native AWS Bedrock (#24), replacing the stub. Run Bedrock-hosted models through RiskKernel with the same budgets / approvals / audit / OTel as any other provider.

export AWS_ACCESS_KEY_ID=...  AWS_SECRET_ACCESS_KEY=...  AWS_REGION=us-east-1
export RISKKERNEL_DEFAULT_PROVIDER=bedrock
riskkernel serve
# then call with a Bedrock model id, e.g. anthropic.claude-3-5-sonnet-20240620-v1:0

Approach — minimal deps

• Built against the Bedrock Runtime Converse API (unified across models, returns token usage).
• Signed with a hand-rolled AWS SigV4 signer — no aws-sdk-go-v2 dependency (it pulls dozens of modules; this is ~120 lines). Keeps the dependency graph minimal per the project's posture.
• The signer is verified against AWS's published SigV4 get-vanilla known-answer vector — I independently reproduced that vector's signature with openssl before encoding it as the test oracle, so the test isn't circular.
• Bedrock model ids contain : (e.g. …-v1:0); the path is AWS-encoded (%3A) for both the wire request and the signed canonical URI so they match (verified Go's Path/RawPath round-trip).

Wiring

• Registered only when AWS_ACCESS_KEY_ID + AWS_SECRET_ACCESS_KEY are set; with RISKKERNEL_DEFAULT_PROVIDER=bedrock and no creds, the daemon refuses to start with a clear error.
• RISKKERNEL_BEDROCK_BASE_URL overrides the endpoint (VPC/PrivateLink or a mock). AWS_SESSION_TOKEN supported for STS temp creds. Creds from env only, never stored/logged.
• Routing: Bedrock model ids fall to the default provider (they don't match the claude-*/gpt-* prefixes), which is why you set RISKKERNEL_DEFAULT_PROVIDER=bedrock — documented.
• Removes the old stub + the now-unused ErrNotImplemented; no stubs remain.

Honesty

Long-tail Bedrock model ids meter priced:false (cost $0, token budget only) until a RISKKERNEL_PRICING_FILE rate is added — same caveat as the LiteLLM path, documented in docs/PROVIDERS.md.

Tests / verification

• SigV4 known-answer vector; AWS URI path encoding; Bedrock provider against a mock (request shape, well-formed …/bedrock/aws4_request auth header, content-type;host;x-amz-date signed headers, session token in signed headers, response parsing, API-error / missing-creds / missing-region / context-cancel).
• go test -race ./... green; go vet / gofmt clean.
• End-to-end: riskkernel chat --provider bedrock against a mock that validates the SigV4 Authorization header returned signed-ok and parsed usage — proving the env→config→register→sign→call path.

Docs: native AWS Bedrock section in docs/PROVIDERS.md; README + ROADMAP updated (Bedrock moves to Shipped).

Closes #24