fix(cli): dedupe help environment entries

[EfeDurmaz16]

Summary

• remove duplicate top-level help entries for AGENT_BROWSER_SESSION_NAME, AGENT_BROWSER_STATE_EXPIRE_DAYS, and AGENT_BROWSER_ENCRYPTION_KEY
• add an integration test that runs agent-browser --help and asserts AGENT_BROWSER_/AI_GATEWAY env entries are unique within the Environment section

Context

Issue #1335 reports duplicate AGENT_BROWSER_SESSION_NAME help output. While checking main, the functional env path is already wired through parse_flags -> DaemonOptions -> daemon env, so this PR keeps scope to the reproducible help-output duplication.

Duplicate check

Before implementation I checked open PRs and did not find an existing PR for this help/env dedupe. Nearby covered items include #1363 for viewport config, #1364 for proxy slash, #1362 for XDG paths, #1354 for network route body, and #1289/#1315 for comma args.

Tests

• cargo fmt --manifest-path cli/Cargo.toml -- --check
• cargo test --manifest-path cli/Cargo.toml --test help_cli
• cargo test --manifest-path cli/Cargo.toml output::tests

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