v0.29.0: #28 Parts 2+3 — create-RFC offers + contribute-to-pending requests

Extends the v0.26.0 (#28 Part 1) read-time scanner into three buckets in
one pass — active link (Part 1), pending-RFC contribute offer (Part 3),
create-RFC offer (Part 2) — precedence active > pending > candidate. The
backend still emits only structured segments (never HTML), so the surface
stays XSS-safe by construction.

Part 2 — create-RFC offers: a multi-word tag from the #27 taxonomy with no
defining RFC renders, for a create-rights viewer, as an inline "+ create
RFC" affordance that opens the propose modal pre-filled (?propose=<term>;
ProposeModal gained initialTitle). Conservative multi-word gate; broader
heuristics + the Haiku path are deferred.

Part 3 — contribute-to-pending offers: a term matching a super-draft
renders, for a signed-in non-owner, an "ask to contribute" affordance with
the owner's display name. It opens a 3-field request form (who/why/optional
use-case); submitting lands a contribution_requests row (migration 024) and
one actionable §15 notification per owner (new kind
contribution_request_on_pending_rfc, personal-direct). The owner's inbox
shows who/why/use-case inline with Accept/Decline. Accept fires #12's
owner-invite flow with the requester as invitee and echoes a notification
back; decline notifies the requester. Pre-merge idea PRs are out of scope.

New endpoints: GET /api/rfcs/{slug}/contribution-target,
POST /api/rfcs/{slug}/contribution-requests,
.../{id}/accept, .../{id}/decline. The invite issue path was refactored
into one reusable api_invitations.issue_invitation(...) chokepoint shared
by the manual invite endpoint and Part 3's accept.

Tests: 9 new (3 scanner-bucket unit + 6 e2e). Full suite 374 passing.

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

frontend/src/App.jsx

@@ -1,5 +1,5 @@
 import { useEffect, useRef, useState } from 'react'
-import { Routes, Route, Link, Navigate, useLocation, useNavigate } from 'react-router-dom'
+import { Routes, Route, Link, Navigate, useLocation, useNavigate, useSearchParams } from 'react-router-dom'
 import { getMe, subscribeToNotifications } from './api'
 import { anonymize, EVENTS, identify, track } from './lib/analytics'
 import { useLastState } from './lib/useLastState'
@@ -9,6 +9,7 @@ import RFCView from './components/RFCView.jsx'
 import PRView from './components/PRView.jsx'
 import ProposalView from './components/ProposalView.jsx'
 import ProposeModal from './components/ProposeModal.jsx'
+import ContributeRequestForm from './components/ContributeRequestForm.jsx'
 import Landing from './components/Landing.jsx'
 import Login from './components/Login.jsx'
 import BetaPending from './components/BetaPending.jsx'
@@ -51,6 +52,19 @@ export default function App() {
   const [identifyReady, setIdentifyReady] = useState(false)
   const navigate = useNavigate()
   const location = useLocation()
+  // #28 Parts 2–3: the LinkedText create/contribute affordances route via
+  // query params so they need no prop-threading from deep in a comment
+  // list. `?propose=<term>` opens the propose modal pre-filled;
+  // `?contribute=<slug>&term=<term>` opens the contribute-request form.
+  const [searchParams, setSearchParams] = useSearchParams()
+  const proposeParam = searchParams.get('propose')
+  const contributeSlug = searchParams.get('contribute')
+  const contributeTerm = searchParams.get('term')
+  const clearParams = (...keys) => {
+    const next = new URLSearchParams(searchParams)
+    keys.forEach(k => next.delete(k))
+    setSearchParams(next, { replace: true })
+  }
   // v0.15.0 — Page Viewed event taxonomy. We fire on every
   // route change; the analytics wrapper itself decides whether
   // anything ships out (consent + key check). The first fire is
@@ -313,17 +327,26 @@ export default function App() {
           } />
         </Routes>
       </div>
-      {proposeOpen && viewer && (
+      {(proposeOpen || proposeParam != null) && viewer && (
         <ProposeModal
           viewer={viewer}
-          onClose={() => setProposeOpen(false)}
+          initialTitle={proposeParam || ''}
+          onClose={() => { setProposeOpen(false); clearParams('propose') }}
           onSubmitted={({ pr_number }) => {
             setProposeOpen(false)
+            clearParams('propose')
             setCatalogVersion(v => v + 1)
             navigate(`/proposals/${pr_number}`)
           }}
         />
       )}
+      {contributeSlug && viewer && (
+        <ContributeRequestForm
+          slug={contributeSlug}
+          term={contributeTerm || ''}
+          onClose={() => clearParams('contribute', 'term')}
+        />
+      )}
       {inboxOpen && viewer && (
         <Inbox onClose={() => setInboxOpen(false)} lastChangeTick={inboxTick} />
       )}