feat: projects command — find the project IDs the config is missing

Reading gizmo_id during normalization fixed attribution, but it cannot
answer "which projects am I missing?": normalization only runs on
conversations being exported, and a normal run skips everything already
cached. Discovering the gaps would have meant --force re-exporting the
whole archive.

`ai-chat-exporter projects` does it directly. It lists conversations,
collects the g-p- ids they belong to, resolves display names, and prints a
table marking which are absent from .env plus a paste-ready
CHATGPT_PROJECT_IDS line. --write applies it; --deep falls back to one
detail request per conversation when the listing does not carry gizmo_id
(unverified which shape this account returns, so the command reports which
path it took rather than assuming).

This matters beyond tidiness: attribution is now self-correcting, but the
listing pass still needs the ids. Conversations that live only inside a
project never appear in the default listing, so an unlisted project's chats
are not merely misfiled — they are never fetched.

6 CLI tests: reporting, the already-configured case, the g-p- guard, --deep,
the hint when --deep is needed, and --write. 324 pass.

README.md

@@ -201,6 +201,28 @@ ChatGPT project conversations are stored separately from your main conversation
 
 ### Finding your project IDs
 
+The quickest way is to let the exporter find them:
+
+```
+ai-chat-exporter projects
+```
+
+It lists every project your conversations actually belong to, marks which are
+missing from `.env`, and prints a paste-ready `CHATGPT_PROJECT_IDS=` line
+(`--write` updates `.env` for you). If your ChatGPT account does not include
+the project on conversation summaries, add `--deep` and it reads each
+conversation's detail instead — slower, one request per conversation, but
+complete.
+
+Why it matters: project attribution is resolved from each conversation's own
+`gizmo_id`, so exports file correctly whether or not a project is configured.
+But the *listing* pass still needs `CHATGPT_PROJECT_IDS` — conversations that
+live only inside a project never appear in the default conversation list, so an
+unlisted project's chats are never fetched at all. Every export run also names
+any unconfigured project it encounters.
+
+To find them by hand instead:
+
 1. Open ChatGPT and click a Project in the left sidebar
 2. Look at the browser URL — it will look like:
    `https://chatgpt.com/g/g-p-68c2b2b3037c8191890036fb4ae3ed9f-my-project/project`