feat(session): support unfocused agent windows

[alectimison-maker]

Summary

• add bsk session start --no-focus so Agent Windows can open without stealing focus
• carry an optional focused value through the CLI, daemon, protocol, and extension
• preserve the existing focused default when the field is omitted
• document the flag and cover both default and unfocused behavior at each boundary

Closes #26.

Why

bsk session start currently hardcodes focused: true at the Chrome window boundary. That interrupts users who want an agent session to start while they continue working in another window.

The new flag is opt-in. Existing callers omit focused, and the extension continues to default it to true.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• corepack pnpm lint
• corepack pnpm --filter @browser-skill/extension compile
• corepack pnpm --filter @browser-skill/extension test (44 files, 497 tests)
• corepack pnpm --filter @browser-skill/extension build
• node --test scripts/*.test.mjs

Compatibility

The protocol field is optional and omitted by default. Older clients therefore keep the current focused behavior, while --no-focus explicitly sends focused: false.