project: Support env-configured custom agent servers

[odvcencio]

Summary

Add environment-driven registration for a custom agent server. If ZED_AGENT_SERVER_BIN is set, Zed registers a local agent server alongside built-ins. Optional envs: ZED_AGENT_SERVER_ARGS (shlex-parsed) and ZED_AGENT_SERVER_NAME (label). Tilde expansion is supported.

Motivation

• Faster local/internal agent workflows without editing settings or packaging an extension
• Point Zed at an existing ACP-compatible agent binary with minimal setup
• Keep built-ins untouched; opt-in via env vars

Testing

• cargo clippy -p project --all-targets --all-features -- -D warnings (local)
• cargo test -p project env_agent_server_config_parses (local)

Release Notes:

• Added environment-driven registration for a custom agent server via ZED_AGENT_SERVER_BIN/ARGS/NAME so users can point Zed at a local ACP-compatible binary without editing settings.

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[odvcencio]

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