…#117, Gridea-Pro#119, Gridea-Pro#118)

Three related bugs in the Gridea Pro MCP server's post tools:

1. delete_post returned success without deleting the file (Gridea-Pro#117):
   - post_repo.go:Delete blindly appended '.md' to the filename parameter
   - If the caller passed a name with '.md' (e.g. 'xxx.md'), the actual
     os.Remove target was 'xxx.md.md' (didn't exist), the os.IsNotExist
     error was silently suppressed, and the function returned nil.
   - Fix: normalize the filename at the MCP entry point.

2. update_post could silently target the wrong file (Gridea-Pro#119):
   - The repo layer's cache stores FileName without '.md' (trimmed on read)
     but writes the file as FileName + '.md'.
   - createPostHandler didn't normalize the fileName parameter, so a caller
     passing 'xxx.md' would write 'xxx.md.md' to disk while the cache kept
     'xxx.md' — permanently desynced.
   - Subsequent get_post / update_post / delete_post with 'xxx.md' would
     match the desynced cache entry, not the real 'xxx.md' file.
   - Fix: normalize at the entry of all 4 post handlers.

3. Tool schema descriptions mismatched implementation (Gridea-Pro#118):
   - get_post example 'hello-world.md' misled callers into passing
     '.md' (which then triggered bugs Gridea-Pro#117 and Gridea-Pro#119).
   - create_post tags description 'Comma separated tags or JSON array
     string' suggested JSON-array support that the handler never had.
   - README example used 'gridea-pro --mcp' (which doesn't exist) and
     'GRIDEA_SITE_DIR' (actual env var is SOURCE_DIR).
   - Fix: clarify all schema descriptions; correct README config example;
     add note about out-of-order MCP responses (a mark3labs/mcp-go
     stdio transport behavior, not a Gridea bug, but undocumented).

Approach: added a normalizeFileName helper and called it at the entry
of all 4 post handlers (create / get / update / delete). The helper
strips any trailing '.md' so callers can pass either 'hello-world' or
'hello-world.md' interchangeably. The repo layer is unchanged — it
already has the correct invariant (FileName without '.md', disk file
as FileName + '.md'); the bug was at the MCP boundary.

Added tool_post_test.go with 9 cases for normalizeFileName, including
the legacy 'xxx.md.md' migration case (a caller that hit the bug
before this fix).

End-to-end verification: built binary with the patch, ran a Python
script that:
  - Creates a post with fileName='t1', then calls delete_post with
    filename='t1.md' (the exact bug pattern) — file is now actually
    deleted (previously: lied).
  - Creates a post with fileName='t2', then calls update_post with
    filename='t2.md' (the exact bug pattern) — title updates on the
    correct file (previously: silent miss).

Both pass.