feat(lessons): a lesson is readable, writable and browsable by a human (#3734)

Step 7's actual UI. Before this the frontend had zero lesson code — the kind
existed for agents only, which is rule 27 failing.

THE EDITOR ASKS FOR THE TRIGGER BY NAME, and leads with it. Three fields —
the trigger, the claim, the detail — never one markdown box. That is the
design step 1 settled, and the evidence is blunt: the snippet corpus carries
a trigger on every record with no guard anywhere, because a service composes
the title from a named parameter. What is at 100% is a named structured
field, not a writer remembering a convention. The trigger gets the most room,
its own explanation, and a save button that refuses without it and says why.

The form shows the composed title live, so the writer is agreeing to a
document they can read rather than one assembled out of sight. A 409 from the
duplicate gate is rendered as the record that already covers the moment, with
a link to improve it and an explicit override — not as a failure.

THE BROWSE VOCABULARY GAINS THE KIND, which #3161 warned this step not to get
wrong: a facet chip, a badge label, and routing to `/lessons/:id` rather than
the note editor, which cannot edit a trigger. The badge is neutral alongside
snippet and process — a hue would make the softest record in the corpus look
like the loudest, next to a rule that actually binds.

BOTH DIRECTIONS OF THE PROVENANCE. The detail page resolves `learned_from` to
titles rather than bare ids, because "#4181" tells a reader nothing about
whether it is worth opening. And `LessonsTaughtPanel` answers the reverse on
the record's own page — the direction the task body calls the one that gets
forgotten. It has no author to type it, which is exactly why it tends never
to get built. A component, not markup in the task editor, so the same panel
mounts on any record a lesson can cite instead of being written a second time
(#3207). Silent when empty: most records taught no lesson, and a panel that
says "None yet" everywhere is one people learn to skip.

GLOBAL-BY-DEFAULT IS MADE LEGIBLE. A lesson meeting you on a project it was
not written on reads as a bug unless the page says otherwise, so the origin
line says it as a property of the kind rather than as an apology.

Design system tokens throughout; no new raw hex. `--fs-error` rather than
`--fs-danger` — 31 uses against 1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

frontend/src/router/index.ts

@@ -69,6 +69,25 @@ const router = createRouter({
       name: "note-edit",
       component: () => import("@/views/NoteEditorView.vue"),
     },
+    {
+      // Lessons have no list view of their own: the Knowledge browse surface
+      // is where every kind is enumerated, and a second list would be a
+      // second vocabulary to keep in step with it (#3128's defect, in
+      // advance). `/knowledge?type=lesson` is the list.
+      path: "/lessons/new",
+      name: "lesson-new",
+      component: () => import("@/views/LessonEditorView.vue"),
+    },
+    {
+      path: "/lessons/:id",
+      name: "lesson-view",
+      component: () => import("@/views/LessonDetailView.vue"),
+    },
+    {
+      path: "/lessons/:id/edit",
+      name: "lesson-edit",
+      component: () => import("@/views/LessonEditorView.vue"),
+    },
     {
       path: "/snippets",
       name: "snippets",