fix: untrack zellij plugin build dir; installer updates all PATH copies

- Remove zellij_socktop_plugin/target from git (3,577 files committed by
  accident in bf6ac87): the root .gitignore anchors /target to the repo
  root, so the standalone plugin's own build dir wasn't covered. Ignore
  target/ at any depth (also fixes the pre-existing
  '/socktop-wasm-test/target' entry, which pointed at a hyphenated path
  that doesn't exist).

- install.sh now updates EVERY copy of socktop/socktop_agent on PATH,
  not just $PREFIX: a stale 'cargo install' in ~/.cargo/bin shadows
  /usr/local/bin on most PATHs, so an install could 'succeed' while
  'socktop --version' kept reporting the old release. Extra copies that
  can't be written are warned about, not fatal, and the script now
  prints which binary is actually active on PATH at the end.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,7 @@
-/target
+# Any crate's build directory, including standalone sub-crates
+# (zellij_socktop_plugin, socktop_wasm_test) that live outside the workspace.
+target/
 .vscode/
-/socktop-wasm-test/target
 /.cargo/
 
 # Documentation files from development sessions (context-specific, not for public repo)