refactor(sidebar): collapse rail/sections to single-IA, log docs

- Sidebar: kill the drifting railGroups + sections dual definition.
  Single source of truth (workItems / libraryItems / footerItems)
  rendered in both pinned and rail modes; pin/unpin is a width and
  label affordance, not an IA switch. Hairline divider replaces
  section labels. Guides moves to the footer alongside Account.
  Renames: Home -> Dashboard, History -> Sessions, Insights -> Analytics.
- CURRENT-STATE.md: log PR #158 (session impeccable pass + tasklane
  keyboard flow) under "Recently shipped".
- PRODUCT.md: design-context source of truth (users, brand, aesthetic);
  sibling to DESIGN-SYSTEM.md.
- skills-lock.json: lock /impeccable + /documentation-writer skill
  versions so other sessions reproduce the same tooling state.
- Drop stale .impeccable.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

skills-lock.json

@@ -0,0 +1,17 @@
+{
+  "version": 1,
+  "skills": {
+    "documentation-writer": {
+      "source": "github/awesome-copilot",
+      "sourceType": "github",
+      "skillPath": "skills/documentation-writer/SKILL.md",
+      "computedHash": "ee53d65b163cd7eb953a930c95841cfe398cc2c0bd24c06508bbaa07c432be35"
+    },
+    "impeccable": {
+      "source": "pbakaus/impeccable",
+      "sourceType": "github",
+      "skillPath": ".agents/skills/impeccable/SKILL.md",
+      "computedHash": "de38608ceb9573c3142306babd9057a240791c4e4d6647f874a4650c996cb37e"
+    }
+  }
+}