fix(config): rename on-disk config file to splunk-ao-config.json (HYBIM-918)

[etserend]

Summary

• Renames SplunkAOConfig.config_filename from galileo-python-config.json → splunk-ao-config.json (src/splunk_ao/config.py:74)
• Updates migration tool README: removes config filename from the "unchanged" list, adds new §5.3 with old→new table, adds checklist item in §9
• Adds breaking change entry to CHANGELOG.md

Breaking Change

Users with saved credentials at ~/.galileo/galileo-python-config.json must either rename the file to ~/.galileo/splunk-ao-config.json or delete it and re-authenticate.

Test plan

• grep confirms no tests reference the old filename — no test changes needed
• 1902 tests passed locally (poetry run pytest); skipped tests are pre-existing optional-dep issues unrelated to this change

Jira: HYBIM-918

🤖 Generated with Claude Code

[fercor-cisco]

🤖 This review was generated by the Astra agent (claude-opus-4-8). It may contain mistakes.

Verdict: approve — Correct, well-scoped rename matching the ticket; docs, changelog, and a regression test are all consistent with no stale references.

Follow-ups

Suggested follow-up work that could be tracked as Jira tickets:

• src/splunk_ao/config.py:74-74: The config directory remains ~/.galileo/ (still using the old brand name) while only the filename is rebranded to splunk-ao-config.json. Consider a separate ticket to evaluate renaming the directory for brand consistency, if desired.

[fercor-cisco]

Follow-up ticket created: https://splunk.atlassian.net/browse/HYBIM-952

[fercor-cisco]

CHANGELOG.md contains a change unrelated to this PR.

After the rebase onto current `main`, the `CHANGELOG.md` hunk re-adds the entire `### Breaking Changes` section — which includes a second bullet that is out of scope for HYBIM-918:

```

• `SplunkAOLogger.flush()` and `async_flush()` now return `None` and only drain
  completed spans already queued for OTLP export. They no longer conclude open
  spans or return uploaded proprietary trace objects.
  ```

That `flush()`/`async_flush()` entry was deliberately removed from `main` in the release-refresh commit (`acc2d24`) — it no longer exists anywhere in `main`'s CHANGELOG. Re-introducing it here would silently revert that intentional edit.

This PR's CHANGELOG change should contain only the config-file rename entry:

```

Breaking Changes

• Config file renamed (HYBIM-918): The on-disk credentials file written to
  `~/.galileo/` has been renamed from `galileo-python-config.json` to
  `splunk-ao-config.json`. ...
  ```

Could you drop the `flush()` bullet from the CHANGELOG hunk? The other three files (`config.py`, `test_config.py`, migration-tool `README.md`) look clean.