[P4-11] graff conversation export <cid> (JSONL) + graff doctor

[justrach]

Parent: #34
Priority: P4 (CLI polish — Hermes-parity quality of life)
Branch base: release/0.1.53

Why

graff-agent's gateway/cron sidecars need to dump conversations as JSONL transcripts (Hermes uses ~/.hermes/sessions/). And hermes doctor is a beloved feature — checks provider auth, MCP servers, FS perms.

Scope

graff conversation export <cid>

• Output JSONL — one line per turn (system / user / assistant / tool_call / tool_result).
• --format json keeps existing single-doc dump; --format jsonl is new (default for export).
• Default output path: ~/.graff/sessions/<cid>.jsonl; --out <path> to override.

graff doctor

• Provider auth check (one round-trip per configured provider).
• MCP server probe (list tools).
• DB integrity check (PRAGMA integrity_check).
• Skills directory readable + parses.
• Optional --fix that writes a sample config when none exists.
• Exit code 0 on all-pass, 1 on any failure; concise human-readable output.

Acceptance

• graff conversation export <cid> round-trips: export → import via --conversation <path> reproduces the conversation
• graff doctor exits non-zero when an API key is missing
• Both commands have --porcelain output for scripting (matches existing graff convention)