BREAKING CHANGE: SDK endpoints updated from enterprise to OSS gateway.

Python SDK:
- agent_client.py: Replace identity-based badge-in with X-Admin-Token auth
  - /v1/mint → POST /api/capability/mint (with X-Admin-Token header)
  - /v1/mint/delegate → POST /api/capability/delegate
  - Add support for pre-existing tokens via 'token' param or SATGATE_TOKEN env
  - Add ping() and validate_token() convenience methods
- client.py: Rewrite for OSS endpoints
  - tokens.mint() → POST /api/capability/mint {scope, duration}
  - tokens.validate() → POST /api/capability/validate
  - tokens.delegate() → POST /api/capability/delegate
  - governance.ban() → POST /api/governance/ban
  - governance.get_graph() → GET /api/governance/graph
  - governance.reset() → POST /api/governance/reset
  - health() → GET /health
  - Remove enterprise-only: ConfigService, StatsService, tokens.list/get/revoke
- langchain.py: Update to use admin_token/token instead of identity providers
- models.py: Simplify to match OSS response shapes, add GraphData model
- __init__.py: Update exports (remove identity providers, add GraphData)

Node SDK:
- client.ts: Rewrite for OSS endpoints (same endpoint changes as Python)
  - Add ping() method for token verification
  - Remove enterprise-only: ConfigService, StatsService, tokens.list/get/revoke
- agent-client.ts: Replace identity-based auth with admin token auth
  - Add adminToken/token options, SATGATE_ADMIN_TOKEN/SATGATE_TOKEN env support
  - Add ping(), validateToken(), delegate() convenience methods
- types.ts: Simplify to match OSS types, add GraphData/GraphStats
- index.ts: Update exports

Docs:
- Add docs/guides/langchain-integration.md with practical examples:
  - SatGateTool single tool usage
  - SatGateToolkit for multiple tools with shared budget
  - Budget-aware agent patterns
  - Delegation for worker agents (least-privilege)
  - L402 Lightning payment integration
  - REST API tool for flexible access
  - Full research agent example

Housekeeping:
- Add .gitignore (sdk/python/venv/, sdk/python/dist/, dist/, node_modules/)
- Remove sdk/python/venv/ and sdk/python/dist/ from git tracking