Remove "Generated by construct" watermark from git commits

[Copilot]

construct was silently appending a Generated by construct trailer to every commit made inside a session via a commit-msg hook injected at container startup. This removes that watermark entirely.

Changes

• internal/runner/runner.go — drop the commit-msg hook block from generatedEntrypoint() (mkdir .githooks, hook script, chmod +x, git config core.hooksPath); clean up the related comment in buildRunArgs
• internal/runner/runner_test.go — replace the two hook-specific tests with a single TestGeneratedEntrypoint_EndsWithExec
• docs/spec/git-identity.md — remove section 3 ("Commit-msg hook") and update the files-changed table
• docs/spec/image-build-layers.md — remove hook step from entrypoint description and hook row from the layer-responsibility table
• docs/spec/construct-agents-md.md — remove hook reference from files-changed table
• docs/threat-model.md — remove mention of the trailer from the attribution note
• CHANGELOG.md — remove the commit-msg hook bullet from [Unreleased]

Original prompt

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This section details on the original issue you should resolve

<issue_title>please remove the watermark we set in git commits</issue_title>
<issue_description>Also check if it is somewhere in the spec.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes please remove the watermark we set in git commits #4

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