feat: add generic CalDAV calendar data source (#42)

* feat: add generic CalDAV calendar data source

Add @aris/source-caldav package that fetches calendar events from any
CalDAV server via tsdav + ical.js.

- Supports Basic auth and OAuth via explicit authMethod discriminant
- serverUrl provided at construction time, not hardcoded
- Optional timeZone for correct local day boundaries
- Credentials cleared from memory after client login
- Failed calendar fetches logged, not silently dropped
- Login promise cached with retry on failure

Co-authored-by: Ona <no-reply@ona.com>

* fix: deduplicate concurrent fetchEvents calls

Co-authored-by: Ona <no-reply@ona.com>

* fix: timezone-aware signals, low-priority cancelled events

- computeSignals uses startOfDay(timeZone) for 'later today' boundary
- Cancelled events get urgency 0.1, excluded from context inProgress/nextEvent

Co-authored-by: Ona <no-reply@ona.com>

---------

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-source-caldav/README.md

@@ -0,0 +1,58 @@
+# @aris/source-caldav
+
+A FeedSource that fetches calendar events from any CalDAV server.
+
+## Usage
+
+```ts
+import { CalDavSource } from "@aris/source-caldav"
+
+// Basic auth (Nextcloud, Radicale, Baikal, iCloud, etc.)
+const source = new CalDavSource({
+	serverUrl: "https://caldav.example.com",
+	authMethod: "basic",
+	username: "user",
+	password: "pass",
+	lookAheadDays: 7, // optional, default: 0 (today only)
+	timeZone: "America/New_York", // optional, default: UTC
+})
+
+// OAuth
+const source = new CalDavSource({
+	serverUrl: "https://caldav.provider.com",
+	authMethod: "oauth",
+	accessToken: "...",
+	refreshToken: "...",
+	tokenUrl: "https://provider.com/oauth/token",
+})
+```
+
+### iCloud
+
+Use your Apple ID email as the username and an [app-specific password](https://support.apple.com/en-us/102654):
+
+```ts
+const source = new CalDavSource({
+	serverUrl: "https://caldav.icloud.com",
+	authMethod: "basic",
+	username: "you@icloud.com",
+	password: "<app-specific-password>",
+})
+```
+
+## Testing
+
+```bash
+bun test
+```
+
+### Live test
+
+`bun run test:live` connects to a real CalDAV server and prints all events to the console. It prompts for:
+
+- **CalDAV server URL** — e.g. `https://caldav.icloud.com`
+- **Username** — your account email
+- **Password** — your password (or app-specific password for iCloud)
+- **Look-ahead days** — how many days beyond today to fetch (default: 0)
+
+The script runs both `fetchContext` and `fetchItems`, printing the calendar context (in-progress events, next event, today's count) followed by each event with its title, time, location, signals, and attendees.