feat(custom-model): Custom Model Endpoint Profiles (local or cloud, all harnesses)

Point any Codeman-supported harness (Claude, opencode, Codex, Gemini, Pi,
Grok, DeepSeek, OMP) at a custom OpenAI-compatible endpoint instead of its
native cloud backend, for a given session. Covers local hardware (llama.cpp,
Ollama, vLLM, DGX Spark, Strix Halo) and cloud (Azure AI Foundry, OpenRouter).
Off by default (customModelEndpointsEnabled, synced, default OFF).

- Registry: capabilities.customModelInjection per CLI entry (env /
  configContentEnv / configDir / unsupported kinds)
- Pure injection builder (custom-model-injection.ts) turning an endpoint +
  model id into the real env vars / config content per CLI
- Endpoint store + CRUD routes (custom-model-hosts.ts,
  custom-model-routes.ts), discovery via GET /v1/models, SSRF-guarded
- Session integration: Session.setCustomModel()/restartCli()
  (POST /api/sessions/:id/custom-model), reusing the existing
  respawn-pane -k primitive to restart the CLI process with new env
- Multi-user hardening: every new redirect-capable env var added to its
  CLI's privilegedEnvKeys, closing a pre-existing gap where several were
  already reachable via the generic envOverrides field's prefix allowlist
- Standalone scripts/test-local-llm-harnesses.mjs: spawns real CLI binaries
  against a real endpoint outside the web UI, independent of tmux/sessions
- Mock-server contract tests (test/fixtures/mock-openai-server.ts) replaying
  every CLI's injected values through a real HTTP shape

Real end-to-end validation against a live llama-swap server (inside a
codeman/agent:llm-test Docker image with all 9 CLI binaries) found and
fixed three real bugs before they shipped:
- Codex's config.toml schema was wrong ([model].default table instead of
  a top-level model string + [model_providers.custom]); fixing it then
  surfaced a genuine, documented protocol incompatibility (Codex only
  speaks the Responses API since Feb 2026, which llama.cpp/llama-swap
  don't implement)
- Claude Code's async session-title-generation call validates
  ANTHROPIC_DEFAULT_HAIKU_MODEL against its own internal model list and
  hangs the whole -p invocation on an unrecognized name; documented for
  chunk 6, worked around in the standalone script only (--bare is NOT
  safe for a real interactive session, which needs hooks)
- The discovery route's authStyle: 'both' option (send both Authorization
  and api-key headers) reliably hung a real server; removed the option
  entirely rather than just changing the default

Status: draft. Chunk 6 (frontend toolbar/settings UI) not yet built — see
PR.md and deployment_plan.md for the full chunk breakdown and confidence
table.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HqNWfmtBU2KN29SvSVWB3

src/types/session.ts

@@ -677,6 +677,13 @@ export interface SessionState {
   resumeSessionId?: string;
   /** Claude CLI effort level (soft default via --settings, switchable in-session via /effort) */
   effort?: EffortLevel;
+  /**
+   * Custom Model Endpoint Profiles (deployment_plan.md): the custom OpenAI-compatible
+   * endpoint (local or cloud) this session's CLI is currently pointed at, if any.
+   * Undefined = the harness's native cloud default. No secrets here — the endpoint's
+   * base URL/api key live only in Session._envOverrides, never in this public state.
+   */
+  customModel?: { endpointId: string; modelId: string; label?: string };
   /** Sanitized per-session attachment history. */
   attachmentHistory?: SessionAttachmentHistoryItem[];
   /**