v0.26.0: auto-link referenced RFCs in PR text + comments (roadmap #28 Part 1)

Part 1 of item #28: references to existing accepted RFCs inside PR
descriptions and comment text now render as inline links to the
referenced RFC. Parts 2 (offer-to-create) and 3 (offer-to-contribute-
to-pending) are deliberately deferred — Part 1 ships first as the easy
win, per the roadmap row.

Shipped in parallel with the v0.25.0 security-hardening session; this
took the next free version slot (0.26.0) per the roadmap's
"claims the next available version number" rule. Expect a top-of-file
CHANGELOG/VERSION merge with 0.25.0 — distinct concerns, trivial to
resolve.

Backend:
- rfc_links.py (new): builds a term index from the live accepted
  (state='active') RFC corpus and segments plain text into text /
  rfc-link segments. Conservative matching — links only rfc_id tokens
  (RFC-0001), multi-word titles (Open Human Model), and hyphenated
  slugs (open-human-model); a single common-word title/slug is NOT
  linked (would turn every prose "human" into a link). Case-insensitive,
  word-boundary-anchored, longest-match-wins, self-reference suppressed.
- api_prs.py get_pr(): enriches the PR description (description_segments)
  and every PR comment (text_segments).
- api_discussion.py: enriches PR-less discussion comments (text_segments).

Read-time, not submit-time: the roadmap says "at submit time" but the
intent it names is "not as live compose preview", which read-time
honors. Chosen for correctness (links track the live active set —
newly-accepted RFCs start linking, withdrawn ones stop), zero migration,
and cheapness (small cache-resident corpus). Recorded as a §19.3-rule-2
note in the session transcript.

Frontend:
- LinkedText.jsx (new): maps backend segments onto React text nodes +
  anchors. No dangerouslySetInnerHTML — XSS-safe by construction,
  independent of any HTML-sanitization layer. Falls back to raw text
  when segments are absent.
- PRView.jsx: description + PR conversation comment bodies render via
  LinkedText.
- RFCDiscussionPanel.jsx: discussion comment bodies render via LinkedText.
- App.css: .rfc-autolink (subtle accent + dotted underline, tokenized).

Tests: 12 new (test_rfc_links_vertical.py) — 9 scanner units + 3
end-to-end (PR description / review comment / discussion comment all
surface *_segments; self-reference suppression). Full suite 363 green;
frontend builds clean.

No upgrade steps: additive, no migration, no secret, no config.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/LinkedText.jsx

@@ -0,0 +1,38 @@
+// LinkedText.jsx — roadmap #28 Part 1.
+//
+// Renders a backend-provided list of text/rfc-link segments (see
+// backend/app/rfc_links.py). RFC references in PR descriptions and
+// comments arrive pre-scanned as structured segments — this component
+// maps them onto plain text runs and anchor elements. It never renders
+// HTML from the server (no dangerouslySetInnerHTML), so the surface is
+// XSS-safe regardless of what a comment author typed.
+//
+// `segments` is the enriched array; `text` is the raw fallback used when
+// the field is absent (an older cached response, or a caller that didn't
+// pass segments). Either way the visible text is identical — only the
+// links differ.
+
+export default function LinkedText({ segments, text }) {
+  if (!Array.isArray(segments) || segments.length === 0) {
+    return <>{text ?? ''}</>
+  }
+  return (
+    <>
+      {segments.map((seg, i) => {
+        if (seg.type === 'rfc') {
+          return (
+            <a
+              key={i}
+              className="rfc-autolink"
+              href={`/rfc/${seg.slug}`}
+              title={seg.title ? `RFC: ${seg.title}` : undefined}
+            >
+              {seg.label}
+            </a>
+          )
+        }
+        return <span key={i}>{seg.text}</span>
+      })}
+    </>
+  )
+}

frontend/src/components/PRView.jsx

@@ -23,6 +23,7 @@ import {
   withdrawPR,
 } from '../api'
 import { EVENTS, track } from '../lib/analytics'
+import LinkedText from './LinkedText'
 
 export default function PRView({ viewer }) {
   const { slug, prNumber: prNumberParam } = useParams()
@@ -218,7 +219,9 @@ export default function PRView({ viewer }) {
             <>
               <h1 className="pr-title">{pr.title}</h1>
               {pr.description && (
-                <p className="pr-description">{pr.description}</p>
+                <p className="pr-description">
+                  <LinkedText segments={pr.description_segments} text={pr.description} />
+                </p>
               )}
               {/* #26: the optional ground-truth use case for this change,
                   captured when the PR was opened. Muted "left blank"
@@ -440,7 +443,9 @@ function PRConversation({ threads, messagesByThread, threadsByKind, seenMsgId })
                 {isNew && <span className="chat-msg-new-pip" title="New since your last visit">●</span>}
               </div>
               {m.quote && <pre className="chat-msg-quote">{m.quote}</pre>}
-              <div className="chat-msg-body">{m.text}</div>
+              <div className="chat-msg-body">
+                <LinkedText segments={m.text_segments} text={m.text} />
+              </div>
             </li>
           )
         })}

frontend/src/components/RFCDiscussionPanel.jsx

@@ -19,6 +19,7 @@ import {
   resolveDiscussionThread,
 } from '../api'
 import { EVENTS, track } from '../lib/analytics'
+import LinkedText from './LinkedText'
 
 export default function RFCDiscussionPanel({ slug, viewer }) {
   const [threads, setThreads] = useState([])
@@ -279,7 +280,9 @@ function DiscussionMessage({ message }) {
       {message.quote && (
         <div className="discussion-message-quote">"{message.quote}"</div>
       )}
-      <div className="discussion-message-body">{message.text}</div>
+      <div className="discussion-message-body">
+        <LinkedText segments={message.text_segments} text={message.text} />
+      </div>
     </div>
   )
 }