feat: add --agent/--model/--session CLI startup commands

[xielixing]

Summary

Adds --agent, --model, and --session CLI startup commands to the BitFun desktop app, enabling users to launch the app pre-configured with a specific agent type, model, and/or session to restore.

Closes #2095

Changes

Rust (desktop app)

• src/apps/desktop/src/lib.rs: Added StartupCommand struct and parse_startup_command() function. The run() method calls it at startup and passes the parsed command through .setup() to create_main_window.
• src/apps/desktop/src/appearance.rs: generate_init_script and create_main_window accept an optional startup_command parameter. Injects window.__BITFUN_STARTUP_COMMAND__ as a JSON constant (4KB max).

TypeScript (web-ui)

• src/web-ui/src/shared/types/global-state.ts: Added StartupCommand interface, consumeBootstrapStartupCommand() one-shot reader, and getStartupCommand() in the globalStateAPI export.
• src/web-ui/src/flow_chat/hooks/useFlowChat.ts: createSession now consumes startup command overrides:
  • --agent: overrides the agent type for the first session
  • --model: overrides the model name for the first session
  • --session: tries to restore an existing session by ID before creating a new one (falls back to new session creation if not found)

Usage

# Start with a specific agent and model
BitFun --agent plan --model gpt-4o

# Restore a previous session
BitFun --session <session-id>

# Combine all three
BitFun --agent agentic --model claude-sonnet-4 --session abc123

Validation

• TypeScript: tsc --noEmit passes
• Rust: cargo check -p bitfun-desktop passes
• Rust tests: cargo test -p bitfun-desktop appearance::tests — 3/3 passed