Support full harness child command overrides

[edwin-zvs]

Summary

• Add AGENTD_*_CMD full command overrides for built-in harness child CLIs
• Preserve existing AGENTD_*_BIN behavior as the fallback
• Apply command prefixes to codex, claude, antigravity, and shell adapters in interactive and headless modes
• Document config examples and add command splitting tests

Examples

[adapters.codex]
env = { AGENTD_CODEX_CMD = "exec codex" }

Supported vars:

• AGENTD_CODEX_CMD
• AGENTD_CLAUDE_CMD
• AGENTD_ANTIGRAVITY_CMD
• AGENTD_SHELL_CMD

Tests

• cargo test -p agentd-protocol parse_command_words
• cargo check -p agentd-adapter-codex -p agentd-adapter-claude -p agentd-adapter-antigravity -p agentd-adapter-shell
• cargo build