fix: add automatic update checks

[LichKing-2234]

Summary

• Add a throttled automatic update notice for interactive human CLI commands.
• Keep machine output clean and allow disabling checks with ATLASSIAN_DISABLE_UPDATE_CHECK=1.
• Document the behavior and add regression coverage for notice, throttle, disable, and failure paths.

Test Plan

• PYTHONPATH=src .venv/bin/python -m pytest -q
• .venv/bin/ruff format --check .
• .venv/bin/ruff check README.md pyproject.toml src tests docs

[Copilot]

Pull request overview

Adds an automatic (throttled) update-notice mechanism to the CLI so interactive human runs can be informed about newer GitHub Releases without polluting machine-readable output, and provides documentation + tests for the behavior.

Changes:

• Introduces a cached/throttled check_for_update_notice() flow that records state in an on-disk JSON cache and supports disabling via ATLASSIAN_DISABLE_UPDATE_CHECK.
• Hooks the update notice into the Typer root callback for interactive sessions (stderr TTY) while skipping machine output modes and the update command group.
• Adds README documentation and regression tests covering notice, throttling, disabling, and failure paths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/atlassian_cli/update.py	Implements the update-check state file, throttling logic, and notice formatting/decision logic.
src/atlassian_cli/cli.py	Emits the update notice for interactive, non-machine output commands (excluding update).
tests/test_update.py	Adds tests for update notice behavior + CLI emission rules.
README.md	Documents the automatic update check behavior and disable flag.

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