Sanitize fetched AGENTS template to avoid duplicated section markers

[kalashnikovisme]

Motivation

• The install generator appended Tramway section markers even when the remote AGENTS template already included them, causing duplicated ## End of Tramway AGENTS.md (and start) markers on repeated runs.

Description

• Sanitize the fetched AGENTS template by adding sanitize_agents_template_body and call it from agents_template_body to strip surrounding Tramway section markers when present.
• Preserve the original content when the remote payload does not include the markers, and keep existing UTF-8 encoding/repair behavior.
• Add a regression spec it 'does not duplicate section markers when remote template already includes them' to spec/tramway/generators/install_generator_spec.rb that simulates a remote template containing the start/end markers and asserts each marker appears only once in the produced AGENTS.md.

Testing

• Attempted to run the generator specs with bundle exec rspec spec/tramway/generators/install_generator_spec.rb, but the rspec executable was unavailable in this environment (bundler error).
• Performed syntax checks with ruby -c lib/generators/tramway/install/install_generator.rb and ruby -c spec/tramway/generators/install_generator_spec.rb, both returned Syntax OK.
• Local unit test added to the generator spec to validate the fix (will run under CI or a developer environment with bundle install).

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Codex Task