README documents get_work_package as a tool, but it isn't implemented (implement or remove from docs)

[AndreaV-Lsi]

Problem

The README documents a get_work_package tool (tool #12 in the catalog), but no such tool is
implemented. The server exposes search_work_packages for lookups; get_work_package exists only
as an internal OpenProjectClient method (used by update_work_package to fetch the lock version),
not as an @mcp.tool.

Surfaced during live baseline testing against the instance (issue #1): a client that tried to call
get_work_package had no such tool and fell back to search_work_packages.

Options (pick one)

1. Implement it — add a thin @mcp.tool async def get_work_package(work_package_id: int) in
   src/tools/work_packages.py that wraps the existing client.get_work_package(...) and formats
   with format_work_package_detail (the formatter already exists and is currently imported-but-
   unused in that module). Matches the README and the natural "get one by id" expectation.
2. Remove it from the docs — drop the get_work_package entry from the README and point users
   to search_work_packages (query by id).

Option 1 is likely the better fit (the formatter and client method already exist, so it's a small,
high-value addition), but either resolves the mismatch.

Scope

• Either src/tools/work_packages.py (+ README catalog) for option 1, or README.md only for
  option 2.

Out of scope

• Broader README renumbering / env-template fixes (tracked in Fix README/doc inconsistencies (duplicate tool numbering, .env.example vs env_example.txt) #4).

Acceptance criteria

• The README tool catalog and the actual registered toolset agree about get_work_package.
• If implemented: the tool returns details for a valid id and a clean error for a missing one;
  README updated; a small test added.

Type: small. Related: #1 (surfaced it), #4 (README fixups). Label: backlog.