Retire datamachine-kimaki-session in favor of native kimaki send --cwd

[chubes4]

Problem

Kimaki 0.13.0 natively supports kimaki send --cwd <path> for existing project subfolders and git worktrees. wp-coding-agents still ships bridges/kimaki/bin/datamachine-kimaki-session, which manually creates a notify-only thread, writes directly to Kimaki SQLite tables, and then sends a follow-up prompt.

That direct DB mutation was a workaround for missing native cwd/worktree routing and should be removed or reduced to a very thin compatibility shim.

Scope

• Replace helper behavior with native kimaki send --channel ... --cwd ... --prompt ... routing.
• Avoid direct writes to Kimaki thread_sessions / thread_worktrees tables.
• Update DMC/README guidance and verification output to prefer native --cwd.
• Remove the helper if no caller still needs its old interface, or keep a small wrapper only when needed for DMC compatibility.

Acceptance criteria

• Existing worktree session handoff goes through Kimaki native --cwd.
• No wp-coding-agents code writes directly to Kimaki internal SQLite tables.
• Tests cover the compatibility wrapper or removal path.
• Documentation no longer teaches the DB-backed helper as the primary route.

AI assistance

This issue was identified during an OpenCode GPT-5.5 review of wp-coding-agents against Kimaki 0.13.0.