feat: add experimental agents support (#22290)

feat: add AI chat system with agent tools and chat UI

Introduce the chatd subsystem and Agents UI for AI-powered chat
within Coder workspaces.

- Add chatd package with chat loop, message compaction, prompt
  management, and LLM provider integration (OpenAI, Anthropic)
- Add agent tools: create workspace, list/read templates, read/write/
  edit files, execute commands
- Add chat API endpoints with streaming, message editing, and
  durable reconnection
- Add database schema and migrations for chats, chat messages, chat
  providers, and chat model configs
- Add RBAC policies and dbauthz enforcement for chat resources
- Add Agents UI pages with conversation timeline, queued messages
  list, diff viewer, and model configuration panel
- Add comprehensive test coverage including coderd integration tests,
  chatd unit tests, and Storybook stories
- Gate feature behind experiments flag

---------

Co-authored-by: Cian Johnston <cian@coder.com>
Co-authored-by: Danielle Maywood <danielle@themaywoods.com>
Co-authored-by: Jeremy Ruppel <jeremy@coder.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/biome_format.sh

@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+if [[ $# -ne 1 ]]; then
+	echo "usage: $0 <path-relative-to-site>" >&2
+	exit 2
+fi
+
+script_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+repo_root=$(cd "$script_dir/.." && pwd)
+target=$1
+
+output_file=$(mktemp)
+trap 'rm -f "$output_file"' EXIT
+
+if (
+	cd "$repo_root/site"
+	pnpm exec biome format --write "$target"
+) >"$output_file" 2>&1; then
+	cat "$output_file"
+	exit 0
+fi
+status=$?
+
+cat "$output_file" >&2
+
+if [[ $status -eq 127 ]] || grep -q "Could not start dynamically linked executable" "$output_file" || grep -q "NixOS cannot run dynamically linked executables" "$output_file"; then
+	echo "WARNING: skipping biome format for '$target' because the biome binary is unavailable in this environment." >&2
+	exit 0
+fi
+
+exit $status