feat: robust HTTP session, proxy validation, lazy pandas; tests and CI

[DivInstance]

Summary:

• Add retries/backoff and connection pooling to improve reliability/perf of HTTP calls.
• Validate --proxy format and fail fast with a helpful message.
• Lazy import pandas only when --xlsx is requested to speed startup and reduce overhead.
• Add test_no_txt_flag to verify --no-txt prevents .txt creation.
• Add CI: lint + unit tests (no online) on PRs/push, nightly online tests, docker build on master/rc.
• Update README: document --no-txt, add Output options section, and include --ignore-exclusions in help block.

Rationale:

• Addresses stability and DX; responds to issues about errors, performance, and discoverability.

Testing:

• Local pytest for non-online suite.
• New unit test for --no-txt.
• CI green on matrix; nightly handles online-only checks.

Links:

Related: #2384, #2369, #2211, #2218