feat(provider): add native Anthropic Messages API adapter (Phase 2)

Adds internal/provider/anthropic, the first genuinely new provider.Provider
implementation on top of Phase 1's provider-neutral tool-use loop, alongside
(not replacing) the existing Docker/CLI-subprocess ContainerRunner path for
the "claude" agent type:

- internal/provider/anthropic: translates the neutral ChatRequest/ChatResponse
  shape to/from Anthropic's Messages API content-block format (system as a
  top-level field, tool_use/tool_result blocks, no "tool" role -- tool
  results become user-role messages), with a per-model-prefix pricing table
  for CostUSD
- internal/retry: IsRateLimitError additively extended to recognize
  Anthropic's rate_limit_error/overloaded_error/529 shapes
- internal/config: RunnersConfig.Anthropic/AnthropicEnabled() gate
- internal/cli/serve.go, run.go: register runners["anthropic"] as a
  NativeRunner when [providers.anthropic].api_key is set and enabled --
  tracked as a distinct executions.agent="anthropic" budget bucket, separate
  from the CLI-subprocess "claude" runner even though both bill the same
  Anthropic account

go build/vet/test -race all pass. No live Anthropic API key is available in
this environment, so verification is via fake-httptest-server adapter tests
(12 cases, incl. multi-turn tool_result round-trip and rate-limit error
matching) plus a Pool/NativeRunner routing test proving agent.type:
"anthropic" actually reaches the new provider. Live end-to-end verification
against the real API is a follow-up once a key is configured.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01V1moSNCJRcP6kykA4tyUSs

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