feat(session): support unfocused agent windows (--no-focus)

[BB-fat]

Supersedes #54, closes #26 — reworked onto current main so it composes with #52 window sizing (options-object instead of colliding positional params). All credit to @alectimison-maker for the original design.

What

bsk session start --no-focus opens the Agent Window in the background without stealing focus from the user's current window.

How it differs from #54

PR #54 threaded focused through the same positional parameter slot that #52 (merged since) now uses for window size. This rework switches the extension-side interfaces to options objects so the two features compose:

• AgentWindowApi.create(url, opts?: { size?: { width, height }; focused?: boolean }) — focused defaults to true, preserving today's behavior; chrome.windows.create receives focused plus the optional size.
• SessionManager.start(sessionId, opts?: { size?; focused? }) — passes the options object straight through.

Chain

• Protocol (crates/bsk-protocol): SessionStartParams.focused: Option<bool> with serde(default, skip_serializing_if), so old daemons/extensions interoperate; JSON schema regenerated via dump-schema.
• CLI (crates/bsk-cli): --no-focus flag on bsk session start; start_session now takes a SessionStartOptions struct (browser/width/height/focused) instead of growing positional args; record start keeps the defaults.
• Daemon IPC: CliSessionStartParams.focused pass-through; start_session takes an AgentWindowOptions { size, focused } struct (also fixes a new clippy::too_many_arguments at 8 args).
• Extension: handleSessionStart validates size as before and forwards { size, focused }; chrome.windows.create gets focused: opts.focused ?? true.

Tests

• Protocol: focused optional-by-default + false round-trip.
• CLI: --no-focus parse test; daemon↔extension IPC round-trip asserts focused: Some(false) reaches tool.session_start.
• Extension vitest: chromeAgentWindowApi.create (default focus / focused: false / size), SessionManager.start options forwarding, handleSessionStart and dispatcher focused: false forwarding. Assertions updated from positional args to the options object.

Verification

• cargo fmt --all, cargo test -p bsk-protocol (115 passed), cargo test -p bsk --lib (226 passed; only the known environment-dependent sync_continues_on_partial_error fails), cargo test -p bsk --test cli_parse --test sessions_ipc (43 passed), cargo clippy -p bsk-protocol -p bsk --all-targets -- -D warnings clean.
• Frontend: pnpm install, wxt prepare, pnpm lint, tsc --noEmit all clean; vitest 567 passed with only the pre-existing React.act .tsx baseline failures (48, identical on pristine main — verified via stash).
• skill/SKILL.md and crates/bsk-cli/skill/SKILL.md updated and verified byte-identical with cmp.