Populate .gitattributes with linguist-generated markers during repo init

Problem

When swamp-managed repositories are used with GitHub PRs, runtime-generated files (data, outputs, definitions-evaluated, etc.) show up expanded in PR diffs. These files are tracked for reconstruction purposes but aren't meant for human review — they add noise and inflate diff stats, making PRs harder to review.

Users currently have to manually create .gitattributes entries with linguist-generated=true to get GitHub to auto-collapse these files.

Proposed Solution

swamp repo init (and swamp repo upgrade) should populate .gitattributes with linguist-generated=true markers for swamp runtime directories, similar to how the managed .gitignore section works today.

Suggested entries:

# BEGIN swamp managed section - DO NOT EDIT
.swamp/definitions-evaluated/** linguist-generated=true
.swamp/data/** linguist-generated=true
.swamp/outputs/** linguist-generated=true
.swamp/ssh/** linguist-generated=true
deno.lock linguist-generated=true
# END swamp managed section

This would cause GitHub (and other platforms using linguist) to:

• Auto-collapse these files in PR diffs
• Exclude them from +/- line count stats
• Still allow reviewers to expand and inspect if needed

User-authored content (extensions, definitions, vault configs, .swamp.yaml) should NOT be marked, so it shows normally for review.

Alternatives Considered

• Gitignoring these files entirely — but swamp expects them tracked for reconstruction
• Manually maintaining .gitattributes — works but is easy to labt and doesn't stay in sync as swamp adds new runtime directories