feat(api): reshape raw events into TimelineItem

GET /v1/events now returns the presentation form rather than raw
upstream payloads. The frontend stays dumb: it renders title /
subtitle / body segments and picks an icon from a small kebab-case
enum. Title and subtitle are arrays of {text} | {text, url} segments
so the UI can interleave plain copy with anchors without parsing.

New entities (in moments-entities):

  TimelineItem        — id, source, action, occurred_at, icon, title, subtitle, body
  TitleSegment        — Text | Link
  TimelineBody        — Markdown | Commits | Links
  CommitSummary       — sha, short_sha, message, url, author
  TimelineIcon        — kebab-case enum; UI falls back to Generic on unknowns

Reshape lives in moments-core::presentation, dispatched by source.
github.rs covers the event types observed on grenade's feed:
PushEvent, PullRequest{,Review,ReviewComment}Event, Issues{,Comment}Event,
Create/Delete/Fork/Watch/Release/CommitComment/PublicEvent.
Anything else falls back to a generic "<action> on <repo>" line.
Other sources (gitea, hg, bugzilla) currently use a stub fallback;
they get their own reshape modules in steps 5 and 6.

4 unit tests cover the load-bearing cases (push commit list, merged
PR icon swap, issue-comment markdown body, unknown-event fallback).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/moments-core/src/presentation.rs

@@ -0,0 +1,31 @@
+//! Reshape raw stored events into the presentation shape consumed by the UI.
+//!
+//! Storage holds the upstream payload verbatim; transformation lives here so
+//! the rendering can evolve without re-fetching upstream data.
+
+use moments_entities::{Event, Source, TimelineIcon, TimelineItem, TitleSegment};
+
+mod github;
+
+pub fn reshape(event: &Event) -> TimelineItem {
+    match event.source {
+        Source::Github => github::reshape(event),
+        Source::Gitea | Source::Hg | Source::Bugzilla => generic_fallback(event),
+    }
+}
+
+fn generic_fallback(event: &Event) -> TimelineItem {
+    TimelineItem {
+        id: event.id.clone(),
+        source: event.source,
+        action: event.action.clone(),
+        occurred_at: event.occurred_at,
+        icon: TimelineIcon::Generic,
+        title: vec![
+            TitleSegment::text(format!("{} from ", event.action)),
+            TitleSegment::text(event.source.as_str().to_string()),
+        ],
+        subtitle: None,
+        body: None,
+    }
+}