fix(chatgpt): discover archived conversations

[solkennedy]

Fixes #52.

What changed

ChatGptStore::discover() only paginated GET /backend-api/conversations?...&order=updated, the same query chatgpt.com's main sidebar uses. That endpoint excludes archived conversations, which live behind a separate is_archived=true filter (mirroring the web app's Settings → Archived Chats panel). Any archived Codex/ChatGPT conversation was invisible to txcript list --from chatgpt / view / MCP list_sessions until manually unarchived.

Split pagination into a list_conversations(is_archived) helper and call it for both the active (is_archived=false) and archived (is_archived=true) states, merging the results.

Before / after

# before: archived conversations never appear
txcript list --from chatgpt   # only non-archived rows

# after: both states are paginated and merged
txcript list --from chatgpt   # active + archived rows

Testing

• Updated live_store_uses_get_only_and_preserves_auth_headers for the extra request.
• Added discover_includes_archived_conversations, asserting both an active-list row and an archived-list row are returned, and that the two requests carry is_archived=false / is_archived=true respectively.
• cargo test -p txcript --lib (61 passed), cargo clippy -p txcript --lib --all-features (clean).

Not affected: the local Codex CLI harness (src/harness/codex.rs) reads ~/.codex/sessions/**/rollout-*.jsonl off disk directly and has no archived concept, so it was never in scope here.

[NishantJoshi00]

LGTM, will merge this when CI passes