feat: SignalSource abstraction — generalize signal ingestion beyond Todoist (#78)

- Add Signal + SignalSource interfaces to packages/shared-types
- TipCandidate.features widened to Record<string,number|boolean> to match Signal
- TodoistSignalSource: encapsulates fetch, cache, 401 handling, bus events, and act()
- SignalAggregator: parallel fan-out across sources with per-source failure isolation
- Recommender refactored to consume Signal[] via aggregator; source action dispatch via aggregator.act()
- ADR-0009: signal normalization strategy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/adr/0009-signal-normalization.md

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+# ADR-0009 — Signal normalization strategy
+
+**Status:** Accepted  
+**Date:** 2026-04-18  
+**Issue:** #78
+
+## Context
+
+The recommender was hard-wired to Todoist: task fetch, cache, and feature extraction lived inside `recommender.ts` with no abstraction boundary. Adding Google Calendar, Apple Health, or manual input sources would have required forking the pipeline per source.
+
+## Decision
+
+Introduce two abstractions in `packages/shared-types`:
+
+```typescript
+interface Signal {
+  id: string;
+  source: string;
+  kind: 'task' | 'event' | 'habit' | 'insight';
+  content: string;
+  metadata: Record<string, unknown>;   // raw source fields, not used by bandit
+  features: Record<string, number | boolean>; // bandit-ready features
+  timestamp: string;
+}
+
+interface SignalSource {
+  readonly id: string;
+  fetchSignals(userId: string): Promise<Signal[]>;
+  act?(userId: string, signalId: string, action: string): Promise<void>;
+}
+```
+
+`SignalAggregator` calls all registered sources in parallel, isolating failures per source.  
+`TodoistSignalSource` moves all Todoist-specific logic (fetch, 401 handling, cache, bus events) out of the recommender route.  
+The recommender maps `Signal[]` → `TipCandidate[]` via a thin adapter and registers action dispatch through the aggregator.
+
+## Consequences
+
+**Good:**
+- Adding a new signal source is a single `aggregator.register(new MySource())` call.
+- `TipCandidate.features` is now `Record<string, number | boolean>`, matching `Signal.features`. Sources control their own feature names; the bandit serialises them as-is.
+- Source failures are isolated: a broken Google Calendar connector does not prevent Todoist signals from reaching the bandit.
+- `act()` on the aggregator routes actions back to the owning source (e.g. marking a Todoist task done), replacing ad-hoc source-specific logic in the feedback handler.
+
+**Trade-offs:**
+- Feature names are no longer compile-time typed. Convention: sources document their feature keys in their class JSDoc. The Python bandit already treated features as an opaque dict.
+- Each source is responsible for its own token lookup (DB access injected via module-level `db`). This is acceptable in a modular monolith; extract to a token vault interface if sources move to separate processes.
+
+## Alternatives considered
+
+**Typed feature schema per source kind** — rejected: would require union types across all sources and a discriminant on every consumer. The bandit doesn't benefit from TypeScript types at runtime.
+
+**Aggregator holds tokens, passes to sources** — rejected: leaks auth concerns into the aggregator. Sources know their own auth requirements.