feat(agent-sdk): add Claude Agent SDK harness

[cognect]

Wrap Anthropic's programmable Claude Code agent (claude-agent-sdk) as a first-class atomr-agents harness — slash commands, subagents, hooks, MCP, permission modes, sessions, custom system prompts, and the built-in tool set, billed against your Anthropic API credits via the bundled claude CLI. Distinct from the server-side Managed Agents API.

Crates / modules

• agent-sdk-core — AgentSdkConfig (mirrors ClaudeAgentOptions), normalized message/result/event schema, the AgentSdkBackend/AgentSdkSession trait seam, and an in-memory MockBackend (network-free tests).
• agent-sdk-harness — orchestrator impl Callable, session registry under an atomic concurrency quota, SpendLedger credit tracking, .claude/ projection, and (feature actor) the interactive session as an atomr_core::actor::Actor. Default permission_mode = bypassPermissions (overridable).
• agent-sdk-harness-web — axum REST + SSE companion.
• py-bindings — PythonAgentSdkBackend + reverse async-iterator bridge (Rust drives the Python __anext__); AgentSdkHarness/Session/Stream pyclasses; invoke_tool. atomr @tool guests + Rust tools bridged in as in-process SDK MCP tools.
• python/atomr_agents/agent_sdk.py — ClaudeAgentSDKBackend wrapper + harness() builder; pip install atomr-agents[agent-sdk].
• host — <root>/agent-sdk/<id>/ loader → spec + .claude projection.
• Umbrella agent-sdk feature; release publish-order updated; docs + CHANGELOG.

Verification

• cargo check --workspace clean; new-crate tests 14 + 8 + 2 pass; actor feature compiles + passes; host loader 2 tests pass.
• py-bindings cdylib builds, links, and imports; Python suite 8 pass (incl. native mock round-trip through the full PyO3 → Rust → MockBackend path); pure-logic tests validate normalization/options. Live SDK test gated on ANTHROPIC_API_KEY + claude CLI.

Note: merging triggers the CI release pipeline, which publishes the three new crates (publish-order updated accordingly).

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