feat(settings): one page per section with a grouped nav and search

Settings moves from a tab bar to /settings/<section>: a left nav grouped
General / Agents / Routing / Git / Projects (a select on narrow screens), a
search box over every section (Ctrl/Cmd+F, Enter opens the first match), and
row primitives. Each AI tool has its page under agents/<id>, the AI Tools
modal becomes the Agents page, Pricing shows every vendor, Git → Worktrees
sets which untracked worktrees the sidebar lists, and each project gets a
page with its own worktree override and Forget all. ?tool= and #ai-tools
links redirect to the path form.

README.md

@@ -56,7 +56,11 @@ Internal build — all rights reserved.
 
 **Configure**
 
-- **Claude Code + Codex configuration explorer.** Settings hosts one panel per
+- **Settings as one page per section** (`/settings/<section>`): a left nav
+  grouped General / Agents / Routing / Git / Projects, a search box (Ctrl+F)
+  over every section, a page per installed AI tool, per-vendor model pricing,
+  which untracked worktrees the sidebar lists, and a page per project.
+- **Claude Code + Codex configuration explorer.** Settings hosts one page per
   tool: every plugin, skill, subagent, slash command, MCP server, hook, settings
   file, memory file, marketplace, keybinding and statusline Claude Code knows
   about — read for all of them, edit/create/delete for the text-file surfaces