docs: install the [agent] extra in README and correct a baseline comment

[Paulo Lacerda (placerda)]

Two text-only corrections found while auditing the published docs. No behavior changes, no new tests needed.

1. README sends readers down an incomplete install

README.md opens with:

python -m pip install agentops-accelerator
agentops init

and then links straight to the Prompt Agent and Hosted Agent tutorials. Both tutorials use commands that live behind the [agent] extra (agentops cockpit, agentops agent serve, the observability commands), so a reader who follows the README verbatim gets an ImportError partway through the tutorial.

[agent] carries fastapi, uvicorn, azure-monitor-query, and the azure-mgmt-* SDKs. agentops init, agentops eval, and agentops doctor work without it, because azure-identity and azure-ai-evaluation are base dependencies. The published tutorials were already corrected to install the extra; this brings the README in line and states which commands need it.

2. A code comment claims --baseline gates, and it does not

The comment above _PR_BASELINE_PATH in src/agentops/services/cicd.py says passing --baseline makes "threshold comparisons run against the committed baseline".

That is not what happens:

# src/agentops/pipeline/orchestrator.py
def exit_code_from(result: RunResult) -> int:
    return 0 if result.summary.overall_passed else 2

overall_passed is decided by the configured thresholds. pipeline/comparison.py produces regressed, current_passed, baseline_passed, and baseline_overall_passed for the report, and none of them reach the exit code. A run that regresses against the baseline but still clears its thresholds exits 0 and the PR gate stays green.

This matters because the comment is what a reader consults when deciding whether committing a baseline buys them a regression gate. It does not. The comment now says so and points at the function that decides.

The same misstatement existed in the published tutorial and was corrected separately in #394.