Genericize: drop pCloud terminology, use TARGET_DIR / target_dir

pCloud was never a hard dependency - the tool writes to any local directory.
Renaming makes the tool clearly generic before publishing.

- PCLOUD_TARGET → TARGET_DIR
- probe_pcloud → probe_target
- place_into_pcloud → place_into_target
- pcloud_target config key → target_dir
- COE_MIRROR_PCLOUD_TARGET env var → COE_MIRROR_TARGET_DIR
- Hardcoded default: ~/pCloudDrive/Hacking/ChurchOfMalware → ~/coe-mirror
- Log messages and docstrings updated
- Tests updated to match

DevTrack #1286

README.md

@@ -1,6 +1,6 @@
 # coe-mirror
 
-Mirror repos from a Forgejo/Gitea instance to a local directory (typically pCloud). Built for `git.churchofmalware.org` but usable against any Forgejo/Gitea host.
+Mirror repos from a Forgejo/Gitea instance to a local directory. Built for `git.churchofmalware.org` but usable against any Forgejo/Gitea host.
 
 ## What it does
 
@@ -36,7 +36,7 @@ Full schema:
 
 ```toml
 api_base      = "https://git.churchofmalware.org"
-pcloud_target = "~/pCloudDrive/Hacking/ChurchOfMalware"
+target_dir = "~/coe-mirror"
 owners        = ["Nightmare_Eclipse", "shai_hulud"]
 # pending_dir = "~/.local/share/coe-mirror/pending"  # optional
 
@@ -52,7 +52,7 @@ room = ""
 | `COE_MIRROR_CONFIG` | Alternate path to config.toml |
 | `COE_MIRROR_API_BASE` | Forgejo/Gitea base URL |
 | `COE_MIRROR_OWNERS` | Comma-separated owners (`"a,b,c"`) |
-| `COE_MIRROR_PCLOUD_TARGET` | Target directory |
+| `COE_MIRROR_TARGET_DIR` | Target directory |
 | `COE_MIRROR_PENDING_DIR` | Local staging dir |
 | `COE_MIRROR_MATRIX_HOMESERVER` | Matrix homeserver URL |
 | `COE_MIRROR_MATRIX_ROOM` | Matrix room ID |