Add markdown-aware unfurl policy for Phase 667 embed depth.

Introduces unfurl-policy.js with code/spoiler/md-link suppression, first-line
wins extraction, and layout gates; wires format-message and index exports plus
unit tests for regression coverage.

Co-authored-by: Cursor <[email protected]>

test/unfurl-policy.test.js

@@ -0,0 +1,57 @@
+'use strict'
+
+const test = require('brittle')
+const {
+  extractUnfurlUrls,
+  extractFirstUnfurlUrl,
+  stripUrlTrailingPunctuation,
+  urlInsideSpoiler,
+  urlPartiallySpoiled,
+  shouldUnfurlMessageLayout
+} = require('../unfurl-policy')
+
+test('suppress inline code URL', (t) => {
+  t.is(extractFirstUnfurlUrl('see `https://example.com/a`'), null)
+})
+
+test('suppress fenced code URL', (t) => {
+  const c = '```\nhttps://example.com/b\n```'
+  t.is(extractFirstUnfurlUrl(c), null)
+})
+
+test('suppress angle bracket URL', (t) => {
+  t.is(extractFirstUnfurlUrl('link <https://example.com/c>'), null)
+})
+
+test('suppress markdown link target', (t) => {
+  t.is(extractFirstUnfurlUrl('[docs](https://example.com/d)'), null)
+})
+
+test('plain URL unfurls', (t) => {
+  t.is(extractFirstUnfurlUrl('hello https://example.com/e world'), 'https://example.com/e')
+})
+
+test('spoiler wraps URL', (t) => {
+  t.is(extractFirstUnfurlUrl('||https://example.com/f||'), null)
+  t.ok(urlInsideSpoiler('||https://example.com/f||', 'https://example.com/f'))
+})
+
+test('partial spoiler overlap blocks unfurl', (t) => {
+  const text = '||https://example.com/g||extra'
+  t.ok(urlPartiallySpoiled(text, 'https://example.com/g', 2, 26))
+})
+
+test('first line wins multi-line', (t) => {
+  const urls = extractUnfurlUrls('line1 https://a.test/x\nline2 https://b.test/y', 3)
+  t.is(urls.length, 1)
+  t.is(urls[0], 'https://a.test/x')
+})
+
+test('strip trailing punctuation', (t) => {
+  t.is(stripUrlTrailingPunctuation('https://example.com/h).'), 'https://example.com/h')
+})
+
+test('emoji-only layout suppresses unfurl policy', (t) => {
+  t.is(shouldUnfurlMessageLayout('emoji-only'), false)
+  t.is(shouldUnfurlMessageLayout('text'), true)
+})