fix: add POST legacy compat routes for v0.18.0 CLI

The v0.18.0 CLI (Homebrew) calls POST /projects/projects for project add
and POST /projects/config/sync for config sync. The previous legacy compat
fix (a0e754b) only added GET for list_projects but missed POST endpoints.

This caused 405 Method Not Allowed when running `bm project add` in cloud mode.

🔗 Logfire trace: trace_id=019c398cb257f040c1255821b6d5e385

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

Author: phernandez