[gadenbuie]

Fixes #225

Summary

path_find_in_project() walked up from the working directory until it found a project root marker (DESCRIPTION, .git, .vscode, .here, or an *.Rproj file) or the filesystem root. The home directory wasn't a stopping point, so when the working directory was under ~ but outside a project, the walk reached home and treated ~/AGENTS.md, ~/CLAUDE.md, or ~/btw.md as project files — and ~/btw.md instructions were then read twice: once as the project file, once as the user-level config.

This PR makes the home directory a hard stop for the upward search:

• path_find_in_project() returns NULL as soon as the walk reaches the home directory — either fs::path_home() or fs::path_home_r(), which differ on Windows — without ever checking the home directory itself. Home-level files are handled only by the user-level lookup (path_find_user()); supporting ~/AGENTS.md there is tracked in the companion issue.
• By design, a home working directory skips all project detection methods: a dotfiles repo with ~/.git or a ~/DESCRIPTION does not make home a project.
• read_btw_file() no longer duplicates instructions when path_btw is set explicitly to the same file as the user-level config, e.g. btw_client(path_btw = "~/.btw/btw.md").
• Fixed a latent bug in the *.Rproj root check: it listed the session's working directory instead of the directory being searched, so a stray *.Rproj in the session cwd could falsely mark unrelated directories as project roots, and roots marked only by an *.Rproj file could be walked past.

New internal helpers path_is_home() and path_same() share path_normalize_for_compare() for consistent path comparison, including Windows case-folding.

Verification

With a ~/AGENTS.md present and the working directory under ~ but outside a project, find_btw_context_file() previously returned ~/AGENTS.md as the project context file; it now returns NULL, and ~/btw.md instructions appear once in the system prompt instead of twice. The repro from the issue (using a temporary HOME):

tmp <- file.path(tempdir(), "fakehome")
dir.create(file.path(tmp, "scratch", "sub"), recursive = TRUE)
writeLines("home btw instructions", file.path(tmp, "btw.md"))
# with fs::path_home()/fs::path_home_r() mocked to `tmp`, from tmp/scratch/sub:
find_btw_context_file(search_user = FALSE)   # NULL (previously ~/AGENTS.md)
read_btw_file()$btw_system_prompt            # "home btw instructions", once not twice

New tests cover the home stop, project roots below home, the Windows split home roots, the *.Rproj fixes, and the path_btw dedup guard; the full test suite passes.