Update docs/help text: init/upgrade/fix no longer create `.github/aw` prompt files

[jeremiah-snee-openx]

PR #14150 removed CLI management of prompt files in .github/aw/. However, multiple docs and help strings still describe init/upgrade/fix as creating or updating those files.

Incorrect wording examples (install.md on main):

• L45–48:
  • “Creates .github/aw/github-agentic-workflows.md…”
  • “Creates workflow management prompts in .github/aw/ directory”
• L63–67: lists .github/aw/github-agentic-workflows.md as created by gh aw init

These are outdated after #14150, which keeps prompt files only in the gh-aw repo and has the agent resolve them directly from GitHub.

Other likely outdated references (repo-wide scan):

• pkg/cli/init_command.go (help text: “creating GitHub Copilot instruction files”, “verifies workflow prompt files exist in .github/aw/”)
• pkg/cli/fix_command.go (help text: “Update prompt and agent files to latest templates”)
• docs/src/content/docs/guides/upgrading.md (upgrade says it updates .github/aw/* prompt files)
• docs/src/content/docs/setup/cli.md (init description says it configures prompt files)
• create.md (mentions .github/aw/github-agentic-workflows.md as a file that may be created by init)

Expected change:

• Update doc/help text to reflect that .github/aw/* prompt files live in the gh-aw repo and are not created/updated by CLI commands.
• Keep mention that init still creates .github/agents/agentic-workflows.agent.md (dispatcher agent) and other local config files.

[pelikhan]

init should write the agent file.

[pelikhan]

Validate that init creates .github/agents/agentic-workflows.agent.md