docs(HYBIM-840): Init splunk-ao-migration-tool - add readme and before/after examples (#71)

* docs(HYBIM-840): add galileo → splunk-ao migration guide

Adds docs/MIGRATION_GUIDE.md covering all breaking changes customers
must make when migrating from the Galileo Python SDK to splunk-ao:

- Package install: not yet on PyPI — GitHub install (Option A) and
  local editable clone (Option B) documented
- Python floor bump: 3.10 → 3.11
- Import path rename: galileo → splunk_ao
- Class/symbol renames (10 public symbols: GalileoLogger,
  GalileoDecorator, galileo_context, exception classes, metric classes,
  handler/middleware classes, ADK classes)
- Satellite package renames: galileo-a2a → splunk-ao-a2a,
  galileo-adk → splunk-ao-adk
- 19 environment variable renames: GALILEO_* → SPLUNK_AO_*
- HTTP tracing header renames: X-Galileo-* → Splunk-AO-*
- Protect feature legacy status (removed from splunk-ao)
- GalileoScorers removal (use SplunkAOMetrics)
- Before/after code examples and a full migration checklist
- "What you do NOT need to change" section

Covers tickets: HYBIM-713, HYBIM-716, HYBIM-717, HYBIM-718, HYBIM-719,
HYBIM-725, HYBIM-727, HYBIM-728, HYBIM-800, HYBIM-804, HYBIM-807,
HYBIM-823.

Co-authored-by: Cursor <cursoragent@cursor.com>

* docs(HYBIM-840): address PR review comments

- Fix §7 code example: project/log_stream are SplunkAOLogger constructor
  args, not start_session kwargs; conclude() has no flush kwarg — use
  logger.flush() separately
- Scope §4 bridge note to the 13 auth/identity vars actually bridged by
  SplunkAOConfig._bridge_env_vars(); clarify remaining vars (LOG_LEVEL,
  LOGGING_DISABLED, scorer/validation vars) are consumed directly
- Document SPLUNK_AO_API_URL as an implicit Pydantic settings field from
  galileo-core (not a user-facing GALILEO_API_URL rename); add AI-agent
  note so future reviewers understand why the row is kept in the table
- Remove phantom HYBIM-729 row from ticket table; the header rename is
  already correctly covered by HYBIM-804

Co-authored-by: Cursor <cursoragent@cursor.com>

* docs: remove Jira ticket references from migration guide

Remove the 'Jira Tickets Covered' section (§8) and all inline
'Related ticket(s)' lines, and the Epic link from the header.
Renumber §9 → §8, §10 → §9 accordingly.

Co-authored-by: Cursor <cursoragent@cursor.com>

* Create new splunk-ao-migration-tool dir

Add README.md

* Address review comments

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Co-authored-by: Cursor <cursoragent@cursor.com>

Author: adityamehra