feat(mcp): facade-only run mode — adapter-less [mcp] config is valid

[chaodu-agent]

Closes #1451, per the #1449 review discussion: the standalone facade entrypoint existed only because openab run refused to start without a chat adapter. One entrypoint, config-driven.

What changes

• openab run with no chat adapter but [mcp] present now serves just the OAB MCP Facade listener in the foreground (the facade is the workload; bind failure/server exit = process exit, fail fast). Log line announces facade-only mode.
• The no adapter configured bail message now mentions the [mcp] option.
• Removes openab-agent mcp-facade — serving surfaces don't belong on the coding-agent binary; shipped only hours ago in feat(mcp): OAB MCP Facade MVP — shared openab-mcp crate + broker loopback HTTP server #1448, nothing depends on it.
• config.toml.example documents the mode.

Who needs facade-only

Coding-CLI-only hosts (no bot anywhere), provider/dev iteration loops (validated tonight — the #1449 e2e had to detour to the agent binary), and CI/agent hosts where a chat token would be absurd. Normal bot deployments are unchanged: [discord]+[mcp] in one process remains the production path.

Validation

• Root workspace: build release, 18 tests pass; openab-agent workspace builds; cargo clippy --workspace --all-features -- -D warnings clean.
• Live functional check (macOS arm64): openab run -c with a two-line [mcp]-only config → facade-only mode + listening logged, MCP initialize on the configured port returns HTTP 200; process killed cleanly.

Review Contract

Goal

Make an adapter-less config with [mcp] a valid openab run deployment (facade-only), and remove the misplaced openab-agent mcp-facade serving surface.

Non-goals

• No new CLI subcommands (explicit maintainer direction — config-driven, not CLI-driven).
• No change to the production path ([discord]+[mcp] in-process facade).
• Native-adapter lifecycle management by the broker (config-driven adapter start) — tracked as the follow-up direction on Relocate standalone mcp-facade entrypoint from openab-agent to the broker binary #1451, separate PR.
• openab-agent's client-side mcp CLI (list|status|doctor|connect|login) is untouched.

Accepted Residual Risks

• Anyone who adopted openab-agent mcp-facade in the hours since feat(mcp): OAB MCP Facade MVP — shared openab-mcp crate + broker loopback HTTP server #1448 merged must switch to openab run with an [mcp]-only config (judged: nobody).
• Facade-only mode skips chat-adapter preflights by design; hooks/secrets behavior is identical to a normal run.

Acceptance Criteria

• openab run with [mcp]-only config serves /mcp (initialize succeeds) and logs facade-only mode; with neither adapter nor [mcp], the bail fires with the updated message.
• openab-agent mcp-facade no longer parses; openab-agent mcp … client commands unaffected.
• fmt/clippy/test green on both workspaces.

Follow-ups

• docs: OAB MCP Facade operator guide #1450 doc updated in its own PR branch (same push series) to describe facade-only mode instead of the removed agent subcommand.
• Config-driven native-adapter lifecycle (broker starts gmail-native alongside the facade) — future PR per Relocate standalone mcp-facade entrypoint from openab-agent to the broker binary #1451.