feat(skills): custom skill directories via btw.skills.paths / BTW_SKILLS_PATHS

[gadenbuie]

Summary

Adds a single btw.skills.paths R option and BTW_SKILLS_PATHS environment variable that replace all user-level and project-level skill search directories. Package-bundled skills and skills from attached packages are always preserved.

Key design decisions:

• Replace semantics: when set, the value entirely replaces user + project defaults (no merging). This is simpler to reason about and is the primary use case for agents and CI pointing at a specific skills directory.
• Precedence: option beats envvar beats default.
• Path format: character vector for R options (c("/path/a", "/path/b")), OS-native path separator (: / ;) for the env var. Non-existent paths are silently skipped.
• Registration-time capture: values are read when btw_tools() / btw_client() is called and closed over in the tool's function, so they survive after transient options set by btw_client() go out of scope.
• btw.md support: the new flat key options.skills.paths maps directly to btw.skills.paths.

Verification

# Point the skill tool at a custom directory
withr::with_options(
  list(btw.skills.paths = c("~/my-project/skills", "~/.agents/skills")),
  {
    tools <- btw_tools("btw_tool_skill")
    btw_skills_list()  # should include skills from custom dirs
  }
)

# Or via environment variable
Sys.setenv(BTW_SKILLS_PATHS = "~/.agents/skills")
btw_skills_list()