feat: add install download timeout option

[officialasishkumar]

Summary

Adds a configurable timeout for agent-browser install so users on slow or high-latency networks can extend Chrome for Testing download requests beyond the 120-second default.

Closes #1285.

Changes

• Parse agent-browser install --timeout <seconds>, --timeout=<seconds>, and -t <seconds>
• Apply the timeout to the HTTP client used for Chrome metadata and browser downloads
• Keep the default at 120 seconds and reject missing, non-numeric, or zero timeout values
• Update CLI help, README, docs, and core skill command references

Validation

• cargo fmt --manifest-path cli/Cargo.toml -- --check
• cargo clippy --manifest-path cli/Cargo.toml -- -D warnings
• cargo test --manifest-path cli/Cargo.toml install

Note: a full cargo test --manifest-path cli/Cargo.toml run was started, but did not complete locally because native::parity_tests::test_all_documented_actions_are_handled kept cycling through Chrome launch failure paths in this environment. The process was stopped after the focused install tests and CI formatting/lint checks had passed.

[vercel]

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