fix(cli): show agent list before setting a session model

[tusharmath]

Summary

Introduce AgentInfo — a lightweight agent metadata type — so the command list, agent switching, and agent listing no longer require a fully configured provider/model session to work.

Context

Previously, fetching agents for the command autocomplete list (/agent, /switch, etc.) called get_agents(), which internally resolved the default provider and model from the active session. This meant the command list would fail or be empty whenever no session was configured — a common state when a user first opens a fresh workspace or hasn't yet set a default model. The branch name bug-command-list reflects this: colon-command autocomplete was broken because listing agents required a session that didn't always exist.

Changes

• Added AgentInfo struct to forge_domain with only id, title, and description — the fields needed for listing, no provider/model required
• Added get_agent_infos() to AgentRepository, AgentRegistry, and the API trait alongside the existing get_agents()
• Moved provider/model resolution into ForgeAgentRepository::get_agents() so it happens only when a full Agent is truly needed (e.g., running the agent)
• Replaced all UI-layer calls that only needed agent names/titles (command list, agent switch, show agents, register_agent_commands) to use the new get_agent_infos()
• Removed the services_url parameter from ForgeAPI::init and ForgeInfra::new — the URL is now read directly from config at construction time, eliminating the early URL-validation step in main.rs
• Changed GrpcInfra::channel() to return anyhow::Result<Channel> instead of panicking, propagating URL-parse errors properly
• Updated ForgeGrpcClient to store the server URL as a String (parsed lazily when a channel is first needed) and return Result from channel()

Key Implementation Details

AgentInfo is derived from AgentDefinition before any provider/model injection, so get_agent_infos() never touches the session config. get_agents() still resolves the session in the repository layer (ForgeAgentRepository), keeping that concern out of the service layer (AgentRegistryService).

Use Cases

• Command autocomplete (/agent:, /switch) works immediately on startup even when no default session/provider is configured
• The agents subcommand (/agents listing) no longer errors out on unconfigured workspaces
• Agent registration for slash-command routing uses lightweight metadata, avoiding unnecessary session lookups

Testing

# Run all tests
cargo insta test --accept

# Verify the command list works without a configured session
# Start forge in a fresh directory with no .forge.toml session block and type ":"

Links

• Related PR: fix(cli): show agent list before setting a session model #2910