docs: capture design from walk-with-me (CONTEXT, DESIGN, ADR-0001)

docs/adr/0001-slack-directory-cache.md

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+# Match Members to Slack via a cached whole-workspace directory
+
+We resolve Slack **Avatars** by sweeping the entire workspace once with `users.list`
+and caching an `email → {imageUrl, slackId}` directory in SQLite, rather than calling
+`users.lookupByEmail` per Member. A **Match** is then an in-memory lookup, and an
+unmatched Member is simply one whose email isn't in the directory.
+
+## Considered Options
+
+- **`users.lookupByEmail` per Member** — only fetches who you view, but makes N calls
+  per uncached Project view and is more exposed to per-method rate limits.
+- **`users.list` directory sweep (chosen)** — one paginated sweep covers everyone;
+  repeated Project views cost zero Slack calls; matching is in-memory.
+
+## Consequences
+
+- Cache is a directory snapshot + a single `refreshed_at`, with a **30-day TTL** and a
+  manual **Refresh** (`POST /api/slack/refresh`) — Avatars change rarely, so staleness is
+  cheap and the button covers the exceptions.
+- If the sweep fails, we still return Members (unmatched) and the UI degrades to initials
+  Avatars + a warning — a Slack outage never hides the roster.
+- Deleted/bot Slack users are filtered out of the directory.