Fix find_project_root hijacking git worktrees nested under a Serena project (#1550)

* Fix find_project_root hijacking git worktrees nested under a Serena project

find_project_root searched all ancestor levels for .serena/project.yml
before ever looking for .git. A git worktree whose own .git is a pointer
file, nested under a directory that is an explicit Serena project, was
resolved to the ancestor project instead of the worktree. With CLI agents
that launch via --project-from-cwd (Claude Code, Codex, Gemini), this
silently bound the server to the wrong working tree: reads returned stale
symbols and edits landed in the parent repo.

Walk up in a single pass so the nearest project boundary wins, whether it
is a .serena/project.yml or a .git (dir or worktree/submodule pointer
file). Same-level behavior is unchanged. Adds a regression test.

* Document worktree project-root fix in changelog and usage docs

docs/02-usage/020_running.md

@@ -92,7 +92,9 @@ Some useful options include:
 
   * `--project <path|name>`: specify the project to work on by name or path.
   * `--project-from-cwd`: auto-detect the project from current working directory     
-    (looking for a directory containing `.serena/project.yml` or `.git` in parent directories and activating the containing directory as the project root, if any).
+    (walking up the parent directories and activating the nearest one that contains either `.serena/project.yml`
+    or `.git`, if any). The nearest boundary wins, so a git worktree nested under another Serena project resolves
+    to the worktree itself rather than the ancestor project.
     This option is intended for CLI-based agents like Claude Code, Gemini and Codex, which are typically started from within the project directory
     and which do not change directories during their operation.
   * `--language-backend JetBrains`: use the Serena JetBrains Plugin as the language backend (overriding the default backend configured in the central configuration)