Address remaining README review feedback

[margaretkennedy]

Follow-up to #1, targeting the add-example branch. Addresses the remaining README-related review feedback from @chipkent (does not touch setuptools-deployment.md, which is moving to the core docs PR).

• Server snippets: fixes the duplicated server = server = Server(...) assignment in the root README (two places). Keeps the two-line server = Server(...) / server.start() form rather than server = Server(...).start(), because Server.start() returns None — the two-line form already holds a reference, so the server isn't garbage collected.
• Consistent entry-point naming: renames the my_dh_cli command function app → main to match my_dh_toolkit (cli.py, __main__.py, pyproject.toml, READMEs, and the "Adapt an example" snippet).
• Deferred imports: my_dh_cli/cli.py now has the same comment as the toolkit explaining why deephaven is imported inside the function; Example 2's "What to study" notes it too.
• Readability: splits the long __init__.py explanation in Example 3 into short bullets.
• Redundancy: removes the toolkit summary under the pattern table, which duplicated the Example 3 intro.
• Duplicated library code: Example 3 now states that queries.py/utils.py are deliberate copies of my_dh_library so each example stays self-contained.

Verified the renamed my_dh_cli.cli:main entry point resolves and python -m my_dh_cli --help runs; not run end-to-end against a live Deephaven server.

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