fix(agent): assign dedicated server port to opencode ACP spawns

[leehom0123]

Problem

With the global opencode config pinning server.port (e.g. ~/.config/opencode/opencode.jsonc), the second concurrent OpenCode conversation fails to start with USER_AGENT_STARTUP_FAILED.

Each OpenCode conversation spawns its own opencode acp, which boots an internal HTTP server. The CLI default --port 0 (random) loses to the config value, so the second process fails to bind and exits with ServeError before the ACP initialize handshake completes. An explicit --port does override the config.

Change

pin_opencode_port() appends an OS-allocated free loopback --port to every opencode ACP launch that does not already carry an explicit one. Applied to both spawn paths: the conversation runtime factory and the custom-agent validation probe.

Detection matches the catalog backend (opencode) or the executable name (opencode/opencode.exe, including Windows paths with spaces); other ACP vendors are never touched. User/vendor-specified --port is respected.

Testing

• 4 new unit tests: injection, idempotency, explicit --port respected, non-opencode binaries untouched, Windows paths with spaces.
• Manual e2e on a machine with server.port pinned: two concurrent OpenCode conversations now both reach a healthy runtime (previously the second died with ServeError); opencode custom-agent creation succeeds while the pinned port is occupied.
• cargo test -p aionui-ai-agent --lib: passes except pre-existing Windows-only terminal::tests failures (missing echo/shell binaries), identical on unmodified main.