docs: Reposition around data engineering harness

[mbroecheler]

This implements the repositioning discussed in:
https://docs.google.com/document/d/17q5ooVeT73150bGJg-rWGvKsFuib8nwvaB2daz8WJAU

It is missing the elements listed under "Proposal". Those still need to be implemented.

[github-actions]

Needs improvement — two user-facing documentation paths are broken.

[RISK ASSESSMENT]
MEDIUM: this documentation change affects first-run onboarding and the recovery link printed after a failed CLI test.

Verdict: COMMENT; address the two compatibility regressions before merging. Architectural insight: published documentation routes are public interfaces consumed by both readers and CLI output.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.32%. Comparing base (9c1a8c0) to head (87cf0fa).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2399      +/-   ##
============================================
- Coverage     71.34%   71.32%   -0.02%     
  Complexity     1900     1900              
============================================
  Files           677      677              
  Lines         20356    20356              
  Branches       2553     2553              
============================================
- Hits          14522    14519       -3     
  Misses         4528     4528              
- Partials       1306     1309       +3     

Flag	Coverage Δ
container-e2e	46.77% <ø> (-0.03%)	⬇️
integration	61.99% <ø> (-0.01%)	⬇️

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