feat(dogfood): add gh CLI workflow and GitHub auth for tasks (#21146)

Adds automatic GitHub CLI authentication and workflow instructions to
the dogfood template's Claude Code tasks.

The startup script now authenticates gh CLI using `coder external-auth
access-token github`, eliminating manual authentication. The system
prompt instructs tasks to read GitHub issue details with `gh issue view`
and create feature branches with descriptive names before
implementation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

dogfood/coder/main.tf

@@ -596,6 +596,14 @@ resource "coder_agent" "dev" {
     # Allow synchronization between scripts.
     trap 'touch /tmp/.coder-startup-script.done' EXIT
 
+    # Authenticate GitHub CLI
+    if ! gh auth status >/dev/null 2>&1; then
+      echo "Logging into GitHub CLI…"
+      coder external-auth access-token github | gh auth login --hostname github.com --with-token
+    else
+      echo "Already logged into GitHub CLI."
+    fi
+
     # Increase the shutdown timeout of the docker service for improved cleanup.
     # The 240 was picked as it's lower than the 300 seconds we set for the
     # container shutdown grace period.
@@ -831,6 +839,13 @@ locals {
     -	Built-in tools - use for everything else:
       (file operations, git commands, builds & installs, one-off shell commands)
 
+    -- Workflow --
+    When starting new work:
+    1. If given a GitHub issue URL, use the `gh` CLI to read the full issue details with `gh issue view <issue-number>`.
+    2. Create a feature branch for the work using a descriptive name based on the issue or task.
+       Example: `git checkout -b fix/issue-123-oauth-error` or `git checkout -b feat/add-dark-mode`
+    3. Proceed with implementation following the CLAUDE.md guidelines.
+
     -- Context --
     There is an existing application in the current directory.
     Be sure to read CLAUDE.md before making any changes.