Add AGENTS.md development guide (#4703)

Add a development guide covering repository structure, build/test
commands, and PR checklist to improve developer onboarding and
AI-assisted development workflows.

Task: BABEL-6428

Signed-off-by: Kuntal Ghosh <kuntalgh@amazon.com>
Co-authored-by: Kuntal Ghosh <kuntalgh@amazon.com>

Author: kuntalghosh

AGENTS.md

@@ -0,0 +1,60 @@
+# Babelfish Extensions - Development Guide
+
+## Repository Structure
+
+```
+babelfish_extensions/
+├── contrib/
+│   ├── babelfishpg_tsql/     # T-SQL language support
+│   ├── babelfishpg_tds/      # TDS protocol support
+│   ├── babelfishpg_common/   # Shared data types (geometry, geography, money, etc.)
+│   ├── babelfishpg_money/    # money type
+│   └── babelfishpg_unit/     # Unit testing framework
+├── test/
+│   ├── JDBC/input/           # Primary T-SQL test files
+│   ├── JDBC/expected/        # Expected output files
+│   ├── python/               # Python driver tests + isolation tests (.spec)
+│   ├── dotnet/               # .NET driver tests
+│   └── odbc/                 # ODBC driver tests
+└── dev-tools.sh              # Developer build and test script
+```
+
+## Build and Test
+
+All commands run from the workspace root (parent of both `babelfish_extensions` and `postgresql_modified_for_babelfish`).
+
+### First-time setup
+```bash
+./babelfish_extensions/dev-tools.sh initpg    # Build PostgreSQL engine
+./babelfish_extensions/dev-tools.sh initdb    # Initialize data directory
+./babelfish_extensions/dev-tools.sh initbbf   # Initialize Babelfish
+```
+
+### Daily development
+```bash
+./babelfish_extensions/dev-tools.sh buildbbf  # Build extensions + restart DB
+./babelfish_extensions/dev-tools.sh buildall  # Build PG + extensions + restart DB
+./babelfish_extensions/dev-tools.sh run_pgindent  # Format code (required before PR)
+```
+
+### Running tests
+```bash
+./babelfish_extensions/dev-tools.sh test normal                          # All JDBC tests (multi-db)
+./babelfish_extensions/dev-tools.sh test normal single-db                # Single-db mode
+./babelfish_extensions/dev-tools.sh test upgrade multi-db test/JDBC/upgrade/15_11  # Upgrade tests
+```
+
+### Upgrade testing
+```bash
+./babelfish_extensions/dev-tools.sh minor_version_upgrade SOURCE_WS     # ALTER EXTENSION UPDATE
+./babelfish_extensions/dev-tools.sh pg_upgrade SOURCE_WS TARGET_WS      # Major version upgrade
+```
+
+## PR Checklist
+
+- [ ] `run_pgindent` applied
+- [ ] `test normal` passes in both multi-db and single-db modes
+- [ ] Upgrade tests pass if DDL or catalog changed
+- [ ] `test/python/expected/upgrade_validation/expected_dependency.out` updated if new functions added
+- [ ] PR description includes JIRA link, change summary, test scenarios covered, breaking changes
+- [ ] Two senior engineer approvals obtained