catalog: per-file config (profiles.d/) alongside the monolithic file #28

Surfaced by the design/security review on #25.

catalog install appends to the monolithic ~/.openapi-cli4ai.toml. Per-file profiles (conf.d pattern) reduce blast radius and make install/uninstall/upgrade atomic.

Proposal (no breaking migration)

• Also load ~/.openapi-cli4ai/profiles.d/*.toml with a deterministic merge order; keep reading the monolithic file for back-compat.
• New catalog install writes a per-file entry to profiles.d/.

Acceptance

• Dual-read (monolithic + profiles.d)
• catalog install writes to profiles.d
• Merge/precedence documented

Effort: M · pairs with #32 (uninstall) and config migrate (#33).