refactor(storage): consolidate external session source parsing

[Astro-Han]

Problem

Maka has two user-facing paths over the same Codex and Claude session stores:

• ForeignSession creates a filtered, untrusted handoff for the current task.
• ExternalSession imports a full typed transcript as a new Maka Session.

Their outputs should remain different, but each path currently implements its own discovery, source identity, title and cwd selection, bounded reading, and source-specific parsing. The implementations have already drifted: Codex paths use different presentation records, while Claude handoff does not share the lineage resolution used by full import.

Desired outcome

Use one package-private source catalog and native parser per external source, followed by two explicit projections: safe handoff and full Session import.

The consolidation should:

• select the same source ID, transcript candidate, title, and cwd under the same query policy;
• use one Codex record authority and one Claude lineage resolver before projection;
• preserve the handoff security boundary, redaction, and hostile-data envelope;
• preserve typed full history, externalOrigin, atomic persistence, and replay;
• cover Codex v0.144 and v0.149, Claude rewind and compaction, duplicate IDs, malformed records, and tool call/result pairing with shared fixtures; and
• strictly reduce production code and duplicated tests without adding another facade, compatibility bridge, or generic ExternalMessage model.

Coordinate with #1512 and #3520. Preserve #1512's staleness and hostile-path requirements, and reuse #3520's v0.149 fixture and record-selection fix.

Alternatives or workarounds

Keeping the parsers independent preserves the current duplication and allows source interpretation to drift further. Deriving safe handoff from imported StoredMessage[] would also weaken the explicit trust boundary, so the two projections should remain separate after shared source resolution.

AI-assisted analysis: Codex agents traced both consumers, their security boundaries, related pull requests, and checked-in fixtures. A human contributor must review the final design and implementation.

[CxHsin]

Hi, I would like to take #3625.

I reviewed the current ForeignSession handoff and ExternalSession import paths. They independently perform source discovery, transcript selection, metadata extraction, bounded reading, and source-specific parsing, which has already caused their Codex and Claude behavior to diverge.

My proposed approach is:

1. Base the work on the latest main, while coordinating with feat(core,storage,cli): evidence pointers and staleness flags for foreign session handoff #1512 and fix(storage): import Codex item_completed messages #3520.
2. Introduce one package-private source catalog so both paths resolve the same source id, transcript candidate, title, cwd, and query policy.
3. Keep one native parser per external source:
   • Codex: preserve the event_msg presentation authority and response_item tool authority, including the v0.149 item_completed cases from fix(storage): import Codex item_completed messages #3520.
   • Claude: share lineage resolution for rewinds, compaction boundaries, duplicate records, and malformed records.
4. Keep the two projections explicit and separate:
   • safe handoff with redaction, hostile-data envelope, tool-output exclusion, staleness checks, and bounded reads;
   • full Maka Session import with typed history, tool call/result pairing, externalOrigin, atomic persistence, and replay.
5. Add shared fixtures and parity tests, then remove the duplicated production code and tests.

I will not introduce a generic ExternalMessage model or derive the handoff from imported StoredMessage[], since that would weaken the handoff trust boundary.

One design point I would like to confirm before implementation is how the shared Claude parser should handle lineage when the handoff is restricted to a bounded tail window. If this direction looks reasonable, I can start with the catalog/parity-test seam and submit the work incrementally.

[Astro-Han]

Assigned! btw the Codex session import failed because codex changed their session structure. Make sure to test the Codex session import route! @CxHsin