Add first-class Codewith source discovery with CODEWITH_PATH

[andrei-hasna]

Problem

Sessions only discovers Codex rollouts through CODEX_PATH or ~/.codex/sessions. Codewith rollouts under ~/.codewith/sessions are format-compatible but absent from discovery and provenance.

Expected

Add source=codewith and CODEWITH_PATH, where CODEWITH_PATH points to the Codewith home and defaults to ~/.codewith. Scan ${CODEWITH_PATH}/sessions. Keep CODEX_PATH and source=codex strictly Codex-only.

Acceptance

• Codewith and Codex roots are discovered independently.
• The same native source ID may exist once per source without collision.
• Shared rollout parsing does not conflate provenance.
• Auth-profile and credential metadata are neither ingested nor emitted.
• Fixtures cover default and overridden roots.

Related umbrella: #10
Todo: OPE2-00074