Add embedding seams: BrowserManager::from_client, output capture, pluggable artifact writer

[Glavin001]

Summary

Stacked on #1. Adds the three remaining seams a library host needs to embed the browser-automation core without a filesystem, a spawnable Chrome, or a process stdout. No behavior change on the native CLI path: every seam defaults to exactly what the code did before.

BrowserManager::from_client (native/browser.rs) — the body of connect_cdp_inner after the WebSocket connect, extracted so hosts that build a CdpClient via CdpClient::from_transport (#1) can get a fully initialized manager (target discovery/attach or direct-page setup) without a native WebSocket:

pub async fn from_client(client: Arc<CdpClient>, ws_url: String, direct_page: bool) -> Result<Self, String>

Output capture (output.rs) — all printing in the output module now goes through outln!/out!/errln! macros that write to the process streams by default, or into a buffer while a capture is active:

output::begin_capture();
let resp = execute_command(&cmd, &mut state).await;
output::print_response_with_opts(&resp, action, &opts);
let CapturedOutput { stdout, stderr } = output::end_capture();

This is what gives embedders byte-for-byte CLI stdout parity: they reuse the real formatting code instead of reimplementing it. The macro conversion is mechanical (println! → outln! etc., 139 sites, same format strings).

Pluggable artifact writer (artifacts.rs, new) — screenshot/PDF/HAR/diff-image writes route through artifacts::write(path, bytes), which falls back to std::fs::write unless a host installs a writer via set_artifact_writer (e.g. to persist into object storage on wasm, where std::fs fails at runtime). Path selection and response shapes ({ "path": ... }) are unchanged.

Verification

From cli/: cargo check --all-targets, cargo check --lib --target wasm32-unknown-unknown, cargo clippy --all-targets -- -D warnings, cargo fmt -- --check, cargo test (1065 passed, 0 failed) all green.

Link to Devin session: https://app.devin.ai/sessions/3a5dcc4bfa3d492dbe280f24b4de6cc0
Requested by: @Glavin001

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✅ Devin Review: No Issues Found

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[devin-ai-integration]

Folded into #1 (commits ded7c22/b1c7aa4/07e807f now live on devin/1786050474-wasm-core as 3c715a9/1be33dd + polish). Closing to keep a single agent-browser PR.