[aft-alfonso]

Fixes #746. A project folder that contains linked git worktrees of its own repository (for example under .worktrees/) was walked again for each worktree, so text search, semantic indexing, inspect and the watcher handled tens of thousands of duplicate files and search returned duplicate hits. Ufuk decided to skip them in every walk.

One shared boundary (project_content::NestedWorktreeBoundary) marks a directory as a nested linked worktree when its .git is a file whose gitdir resolves into this repository's common dir under worktrees/. Every content-reading walk prunes it: the trigram build and refresh, semantic enumeration, grep and glob fallback walks, the callgraph walk, inspect's entry-point and resolver walks, and the file watcher. Nested independent repositories and submodules are still walked. A path the agent names explicitly inside such a worktree is still read, the same rule as an explicitly named gitignored path.

Tests use real git fixtures (git init plus git worktree add into a subfolder) for each walk, an independent nested repo, a submodule-style gitdir, a worktree of another repository, and an explicitly named target.