[gadenbuie]

Fixes #226

Summary

When no user-level btw.md exists, btw_client() and btw_app() now fall back to a user-level AGENTS.md in the cross-tool ~/.agents/ directory. This mirrors the project-level precedence btw.md > AGENTS.md > CLAUDE.md, except a user-level CLAUDE.md is never used. Frontmatter in a user-level AGENTS.md configures client, tools, and options just like a btw.md, and user-level context is combined with the project-level context file.

edit_btw_md("user") opens a user-level AGENTS.md when only one exists and includes them in the multi-file menu. use_btw_md("user") warns (or asks interactively) before creating a ~/.btw/btw.md that would take priority over an existing user-level AGENTS.md.

path_btw in btw_client() and btw_app() now also accepts a named list with project and/or user fields, each TRUE (search the default locations), FALSE (skip that scope), or a path to a specific file. A missing field keeps the default for that scope: project searches project locations and user follows the new btw.client.path_btw_user option (TRUE/FALSE/path), so options(btw.client.path_btw_user = FALSE) in ~/.Rprofile opts out of user-level context globally. btw_app()'s chat history scoping understands the list form too.

One deliberate behavior change: a scalar path now uses only that file, a shortcut for list(project = path, user = FALSE). Previously the user-level btw.md was also loaded alongside it; pass list(project = "path") to combine a specific project file with the default user-level context.

Verification

Full test suite passes locally: 2709 pass, 0 fail, 1 skip (CI-only web tool test).

# Project context plus user-level AGENTS.md fallback (the default)
chat <- btw_client()

# Skip user-level context for a single chat
chat <- btw_client(path_btw = list(user = FALSE))

# Use only a specific file (no user-level context)
chat <- btw_client(path_btw = "analysis/btw.md")

# Combine a specific project file with the default user-level context
chat <- btw_client(path_btw = list(project = "analysis/btw.md"))

# Opt out of user-level context everywhere (e.g. in ~/.Rprofile)
options(btw.client.path_btw_user = FALSE)