feat(knowledge): rebuild the project notes panel as Project knowledge (#2973)

The panel stored notes, todos and plans inside one shared JSON file that
six unrelated domains also wrote to, synchronised itself through window
CustomEvents, and could only read plans. It is now Project knowledge:
server-owned storage with explicit routes, a store with rollback, a
section sidebar, plans that open and edit in place, and search across
all of it.

Notes and plans the user pins travel with every message sent in that
project. Pinning is project state, not an attachment to one message, so
it holds until unpinned and the work status panel names what is riding
along and can detach it.

Agent memory is added alongside, in two scopes: what is true about the
user, and what is true about this codebase. The split is not cosmetic —
a wrong project fact costs one project and is noticed, while a wrong
global fact quietly shapes every session everywhere and the user has no
code to check it against. It stays separate from notes so an agent
mistake cannot land in what the user wrote. Sessions receive an index of
titles only; bodies are read on demand, because an index carrying full
text grows until it crowds out the conversation.

Deciding what a session must be told, and whether it has been told, now
lives on the server. The client owned it before, which meant sessions
started without a UI — scheduled tasks, sessions the agent dispatches —
received nothing at all, and a tab's record of what it had sent outlived
the conversation: after compaction the agent no longer held the block
while the tab went on believing it did. What was delivered is recorded
in the session's own metadata, and compaction restores it through the
runtime that already restores pinned messages, in the same turn.

Agent memory ships dark behind OPENCHAMBER_MEMORY_ENABLE: unset, there
is no tool, no routes, no session index, no settings row and no panel
tab. Absent rather than switched off, so nothing invites turning on a
feature that has not been announced. Pinned notes and plans are
unaffected and ship as normal.

packages/web/server/lib/opencode/settings-runtime.js

@@ -212,6 +212,56 @@ export const createSettingsRuntime = (deps) => {
     }
   };
 
+  /**
+   * Merge the server-owned `context.json` (notes/todos/plans) across a project
+   * id change.
+   *
+   * `moveDirectoryContents` only renames a file when the destination is free,
+   * so without this step an existing `<newId>/context.json` would silently
+   * discard everything stored under `<oldId>`. Every list is merged by identity
+   * so neither side loses entries.
+   *
+   * A version 1 context stored notes as a single string. It is left untouched
+   * here: `project-context` converts it on read, and converting in two places
+   * would mean two definitions of the same migration.
+   */
+  const mergeProjectContextFiles = async (oldStorageDir, newStorageDir) => {
+    const oldContextPath = path.join(oldStorageDir, 'context.json');
+    const newContextPath = path.join(newStorageDir, 'context.json');
+
+    const [oldContext, newContext] = await Promise.all([
+      readJsonFile(oldContextPath).catch(() => null),
+      readJsonFile(newContextPath).catch(() => null),
+    ]);
+
+    if (!oldContext || !newContext) {
+      // Nothing to reconcile: the plain directory move handles a single side.
+      return;
+    }
+
+    const mergeNotes = () => {
+      // One side may still be a version 1 string; keep whichever is a list, and
+      // prefer the destination when both are strings.
+      const oldIsList = Array.isArray(oldContext.notes);
+      const newIsList = Array.isArray(newContext.notes);
+      if (oldIsList && newIsList) {
+        return mergeByKey(oldContext.notes, newContext.notes, (item) => item.id);
+      }
+      if (newIsList) return newContext.notes;
+      if (oldIsList) return oldContext.notes;
+      return newContext.notes || oldContext.notes || '';
+    };
+
+    await writeJsonFile(newContextPath, {
+      ...oldContext,
+      ...newContext,
+      notes: mergeNotes(),
+      todos: mergeByKey(oldContext.todos, newContext.todos, (item) => item.id),
+      plans: mergeByKey(oldContext.plans, newContext.plans, (item) => item.id || item.file),
+    });
+    await fsPromises.rm(oldContextPath, { force: true });
+  };
+
   const migrateProjectScopedStorage = async ({ oldId, newId, projectPath }) => {
     if (!oldId || !newId || oldId === newId) {
       return;
@@ -232,6 +282,7 @@ export const createSettingsRuntime = (deps) => {
       await writeJsonFile(newConfigPath, merged);
     }
 
+    await mergeProjectContextFiles(oldStorageDir, newStorageDir);
     await moveDirectoryContents(oldStorageDir, newStorageDir);
     await fsPromises.rm(oldConfigPath, { force: true });
   };