feat(agents): discover custom agents from .btw/agents/ subdirectory

[gadenbuie]

Fixes #202

btw already supports a .btw/skills/ directory, but custom agents could only be placed as flat agent-*.md files. This adds support for an agents/ subdirectory so agent files can be organized in a dedicated folder and copied around more easily.

What changed

• Discovery (R/utils.R): find_project_agent_files() and find_user_agent_files() now also scan an agents/ subdirectory — .btw/agents/*.md at the project level and <user_dir>/agents/*.md at the user level (~/.btw/, ~/.config/btw/, tools::R_user_dir("btw")) — in addition to the existing flat agent-*.md files.
• Naming: files inside agents/ use plain {name}.md names; the agent name is taken from the YAML frontmatter (convention: agents/{name}.md matches the frontmatter name, but the frontmatter is authoritative).
• Precedence: agents/*.md files are returned ahead of flat agent-*.md files, so on a name conflict the subdirectory version wins via the existing first-wins dedup in custom_agent_discover_tools() (which still warns on duplicates). The agents/ directory is otherwise the same priority tier as the flat files.
• Docs: updated the "Automatic Discovery" section of btw_agent_tool() and the project/user location docs in ?btw-config.

Backward compatibility

Fully backward compatible — the flat agent-*.md behavior is unchanged.

Tests

Added tests for project- and user-level agents/ discovery, discovery ordering (subdir before flat), and the agents/-wins-over-flat precedence on a name conflict.

Notes

• No filename/frontmatter-name mismatch warning was added; the frontmatter name remains authoritative and the {name}.md convention is documented in roxygen only.