Promote supported agent adapters

[pentaxis93]

Summary

Closes #184.

• Promotes supported runtime adapters into top-level adapters/.
• Moves the Claude Code adapter out of examples/ and adds agent-codex.sh.
• Documents the supported adapter set and updates stale adapter references.
• Keeps core launch logic runtime-agnostic and tightens the existing invariant fixture so core-path greps are clean.

Verification

This repository has no test-suite CI; only release-metadata/tag-release workflows are expected. I ran the actual gate locally:

• cargo test --workspace -> passed
• cargo fmt --check -> passed
• cargo clippy -> passed
• test -x adapters/agent-claude-code.sh && test -x adapters/agent-codex.sh -> passed
• ! find examples -maxdepth 1 -name 'agent-*.sh' | grep . -> passed
• ! rg "examples/agent-" -> passed
• ! rg -n "claude|codex|--mcp-config|--strict-mcp-config|-c mcp_servers|mcp_servers" libagent runa-cli/src -> passed

Focused checks also run:

• cargo test -p runa-cli --test step adapter -- --nocapture -> passed
• cargo test -p runa-cli commands::step::tests::execute_entry_launches_agent_commands_agnostically -- --nocapture -> passed

Acceptance self-review

• Adapter home exists and contains both supported-runtime adapters: adapters/agent-codex.sh, adapters/agent-claude-code.sh.
• examples/ contains no agent adapter; repo-wide examples/agent- search is clean.
• Codex adapter translation test fails if command, arg order, env entries, forwarded argv, or stdin prompt handling regress.
• Both adapters fail clearly without RUNA_MCP_CONFIG; Codex also fails clearly without jq.
• Core agnostic launch invariant still passes, and core-path grep has no runtime-specific runtime names or MCP flags in libagent/ or runa-cli/src.
• Operator-facing docs name Codex and Claude Code as the supported adapters and instruct operators to point [agent].command at the chosen adapter.