`/changelog.atom` entries carry the wave's headline and link to the wave page, so a feed reader gets the story, not the pull titles #11298

One sentence: the changelog feed lists what went live; where a wave covers it, the entry should speak the wave's headline and link to the wave.

Done:

• [x] An atom entry for a release that has a wave uses the wave's headline as its title and its highlights as the summary, linking to the wave's page; a release without a wave is unchanged
• [x] Every string is XML-escaped; a broken wave file falls back to today's entry, never a 500
• [x] The feed still answers from cache with the same validators and under its current budget
• [x] A test fails without it (wave headline present, escaped, fallback on a broken wave)

Files I expect to change: src/web/feeds.ts, src/story/waves.ts (read only), one test file

Risks: XML escaping of wave text; keep the feed's cache tag honest (the wave's version belongs in it)

Brief that fits: scout. Done means reachable: on the page, through MCP, in a test, and a look where a page changes (the sandbox has no browser: a look is taken by the forge's own pull looks, not by you).

Reviewed pull #11317 at its head: ticked An atom entry for a release that has a wave uses the wave's headline as its title and its highlights as the summary, linking to the wave's page; a release without a wave is unchanged; Every string is XML-escaped; a broken wave file falls back to today's entry, never a 500; The feed still answers from cache with the same validators and under its current budget; A test fails without it (wave headline present, escaped, fallback on a broken wave).