context-graph: propagate resolved Memgraph/LLM config to the reconcile subprocess

[antejavor]

Summary

• SessionsGraphConnector._spawn_reconciliation spawns a detached sessions-graph reconcile subprocess on SESSION_END (when auto_reconcile is on) via a bare subprocess.Popen with no explicit env=. Per ADR 0002, the parent hook process resolves Memgraph credentials from ~/.config/context-graph/config.toml purely as constructor kwargs — never writing them to os.environ — so the spawned subprocess silently fell back to Memgraph defaults, and had no path at all to an LLM API key (LightRAG's default llm_model_func reads OPENAI_API_KEY from os.environ and raises if it's missing).
• Extends the config-file-only resolution mechanism from ADR 0002 with an [llm] section (openai_api_key, anthropic_api_key) and resolve_llm_env(), mirroring resolve_memgraph_env(). bootstrap captures OPENAI_API_KEY/ANTHROPIC_API_KEY from the environment into the config file the same way it already does for Memgraph credentials.
• _spawn_reconciliation now builds an explicit env= for the child: ambient os.environ overlaid with non-empty resolved Memgraph + LLM values, so the detached subprocess gets what the hook process resolved regardless of the harness's own ambient environment.
• sessions-graph reconcile (run standalone via cron/manually) does the same best-effort fill-in at startup via an optional import, so it behaves consistently whether spawned by the hook or invoked directly.
• agent-context-graph config set/get/show and doctor gain the new llm.* keys; command-hooks.md documents them.

Test plan

• uv run --package agent-context-graph --extra test pytest tests/ -v — 83 passed
• uv run --package sessions-graph --extra test --extra agent-context-graph --extra reconciliation pytest tests/ -v — 50 passed, 1 skipped (live-Memgraph e2e test)
• ruff check / ruff format --check on changed files
• Manual smoke test: agent-context-graph config set llm.openai_api_key, config show, config get, and doctor all reflect the new [llm] section correctly in an isolated $HOME