refactor: replace py_jama_rest_client with async httpx client

docs/plans/replace-py-jama-rest-client.md

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+# Plan: Replace py_jama_rest_client with Async JamaHttpClient
+
+## Architecture
+
+```
+Phase 1: New files (no existing code touched)
+  1.1 jama_cli/core/exceptions.py     — exception hierarchy
+  1.2 jama_cli/core/http_client.py    — async httpx client (auth, pagination, retry)
+  1.3 jama_cli/core/api.py            — typed async API methods (~60 wrappers)
+  1.4 jama_cli/core/sync_wrapper.py   — background-thread sync facade for CLI
+
+Phase 2: Update jama_cli/core/client.py  (swap internals, keep public API)
+Phase 3: Update MCP servers             (remove run_in_executor, await directly)
+  3.1 jama_mcp_server/core/stdio_server.py
+  3.2 jama_mcp_server/core/server.py
+Phase 4: pyproject.toml                  (drop py-jama-rest-client, add httpx)
+Phase 5: Update tests
+Phase 6: Cleanup (CLAUDE.md, mypy overrides)
+```
+
+## Key Decisions
+
+- **httpx.AsyncClient** for transport (both sync CLI via thread-bridge and async MCP)
+- **Async-native + background-thread sync wrapper** (one implementation, no duplication)
+- **Preserve exception class names** (APIException, AlreadyExistsException, etc.)
+- **Page size 50** (fix the library's hardcoded 20)
+- **Retry on 429** with exponential backoff (3 retries)
+- **jama_mcp_server imports from jama_cli.core** for shared client layer
+
+## Phases 2+3 are independent and can be parallelized.