feat: session persistence + streaming during --deep (v0.9.0)

[askalf]

Summary

Two features that compose well: every run is now saved to disk, and --deep mode finally streams.

1. Session persistence

Every successful agent run is saved to ~/.deepdive/sessions/<id>.json with the full plan, round trace, kept sources with extracted content, answer, verification report, and cost — enough to re-synthesize without touching the network.

Three new subcommands:

deepdive sessions ls                         # newest first; id, age, source/round counts, question
deepdive show <id>                           # re-print the original markdown answer
deepdive resume <id> [<new-question>]        # re-synthesize against the saved sources

resume is the headline. One LLM call, against the saved source corpus, with the original or a refined question. No re-search, no re-fetch, no critic loop. The cache solved "don't re-fetch URLs"; sessions solve "don't re-run the entire pipeline." Refining a question into a follow-up costs one synthesis call instead of plan + N×search + N×fetch + N×synth + N×critique.

IDs are timestamp-prefixed (YYYY-MM-DD_HHMMSS_<8-hex>), sortable chronologically. resolveSessionId accepts unique prefixes — deepdive resume 2026-05-07_134509 works when only one session matches. After every run a one-line stderr hint prints the id + the resume command.

Atomic .tmp + rename writes mirror the cache module. Persistence failures are non-fatal warnings.

2. Streaming during --deep

Previously auto-disabled because intermediate rounds would print multiple full drafts back-to-back with no separation. v0.9 enables it with a clear UX:

• Round 0 streams under the question's H1 (current single-pass behavior, unchanged).
• Round 1+ each get a \n\n---\n\n## Round N (deep)\n\n separator before tokens start.
• The terminal scrolls naturally as each round's draft writes itself.
• --out=file.md still gets only the final answer; intermediate drafts are visible in the terminal but not persisted there.
• --no-stream and --json continue to suppress streaming.

What's added

• src/sessions.ts (~190 lines, no new runtime deps): generateSessionId, saveSession, loadSession, listSessions, resolveSessionId, renderSessionsList, defaultSessionsDir, humanDuration + SessionRecord / SessionMeta types
• New CLI subcommands: sessions ls, show <id>, resume <id> [<q>] (parseArgs gains a small extras: string[] array, used only when the verb is doctor/sessions/show/resume)
• New CLI flag --no-sessions; env vars DEEPDIVE_NO_SESSIONS, DEEPDIVE_SESSIONS_DIR
• AgentResult.sources type tightened from Source[] to SourceWithContent[] — runtime value was always SourceWithContent, so this is type-only and non-breaking
• RuntimeConfig.sessions: { enabled, dir }
• README "Sessions" section + flag-table row + a paragraph in the --deep section explaining the new round-header streaming UX
• CHANGELOG entry under v0.9.0; package.json bumped

What's explicitly out of scope (v1)

• deepdive resume <id> --continue — running another round of search/critique against an existing session. Defer; the v1 surface is "re-synthesize cheap." Continuation is a coherent v2 feature with its own design conversation.
• Session export/import (compressed bundle for sharing / archival).
• Auto-pruning of old sessions / size cap.
• A --session=<id> flag that resumes inside the main run path (subsumed by the dedicated resume verb for now).

Test plan

• 15 unit tests in test/sessions.test.mjs covering: ID format, ID uniqueness across same-second calls, save/load round trip, atomic write (no .tmp left over), schema rejection, listSessions sorting + bad-file surfacing + missing-dir tolerance, prefix resolution including ambiguity error and no-match error, renderSessionsList empty + non-empty cases
• 6 parseArgs tests: sessions/show/resume verbs land in extras, doctor still works, --no-sessions, sanity check that bare unquoted multi-word questions still throw
• 2 config tests: sessions enabled/disabled, DEEPDIVE_SESSIONS_DIR override
• Existing streaming-config test updated to reflect that --deep no longer disables streaming
• npm run typecheck clean
• npm run build clean
• npm test — 378/378 passing (up from 355)
• CLI smoke test: --help shows new verbs + flag; sessions ls against an empty dir prints the bootstrap hint