feat!: remove standalone env configuration (ACP contract only)

[hallerite]

rlm is now consumed exclusively for training, through the versioned ai.prime.rlm/runtime-v1 ACP contract — and recursive children already inherit configuration in-memory (model_copy), never from the environment. This PR removes the third, now-unused config path: standalone env-var resolution.

Removed

• RuntimeConfig.from_env / ProviderConfig.from_env / InvocationContext.from_env + env parsing helpers — config objects are constructed explicitly (by the ACP layer or by the caller).
• RLMEngine fallback to env: an explicit runtime_config is now required.
• load_mcp_servers / dump_mcp_servers (RLM_MCP_CONFIG): MCP servers come only from the caller/ACP session.
• make_client() env fallback and resolve_provider: an explicit ProviderConfig is required.
• rlm.run() standalone path: it is broker-only (inside a running session); outside one it raises.
• CLI standalone prompt + interactive modes: rlm --acp is the only mode.
• README "Standalone configuration" table → replaced with the contract description; the process env now configures only process infrastructure (RLM_HOME).

Deliberately untouched (internal, not user config): engine→kernel plumbing (RLM_SESSION_DIR, RLM_ALLOW_GIT, kernel-env delivery) and the registry dev presets (RLM_TOOLING, RLM_BUILTIN_TOOLS) — the latter could move into the contract later if wanted.

Tests updated to construct configs explicitly (tests/conftest.py: make_runtime_config); suite green.

Part of the stack: fetch (#132) → contract-only config (this) → execution guardrails (#158) → role-aware sub-agent prompting (#151).

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Note

High Risk
Breaking removal of standalone CLI, SDK entry, and env-based configuration forces all integrators onto the ACP runtime contract; misconfigured clients will fail at engine init rather than silently using ambient env.

Overview
Breaking change: rlm no longer runs from environment variables or a one-shot CLI prompt. Runtime settings must come from the versioned ai.prime.rlm/runtime-v1 object on ACP session/new; recursive children keep inheriting config in memory, not from the process env.

The CLI only accepts rlm --acp. Standalone rlm "prompt", interactive mode, and flags that wrote RLM_* env vars are gone.

rlm.run() and RLMEngine now require an explicit RuntimeConfig (and broker for run()). ProviderConfig.from_env, resolve_provider, load_mcp_servers / RLM_MCP_CONFIG, and the large README env table are removed; MCP servers are passed by the ACP session or the mcp_servers engine kwarg.

Docs are updated so skills, compaction, and kernel env are described against the contract fields (skills, summarize_at_tokens, kernel_env). Tests use make_runtime_config() instead of monkeypatching env.

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