Give the real-git test its own vitest project, and archive the docs change

[VeryComplexAndLongName]

git.push.test.ts spawns around eight real git processes. On Windows it fails intermittently — a 5000 ms timeout, or MSYS's add_item ... errno 1 — and the investigation recorded in core-test-worker-contention traces that to Git for Windows' fork-emulation racing when its subprocesses run concurrently with other git subprocesses.

The file now runs in its own single-fork vitest project rather than sharing a worker with the other 43 files in the package — and rather than forcing all of them onto a single fork, which was tried and rejected for slowing every other file to fix one.

I tried to refute it, and failed

My hypothesis: splitting into two projects changes nothing, because vitest runs projects concurrently — so the isolated file would still race the other 43. First two observations seemed to confirm it (isolated: pass in 2.65 s; full suite: timeout).

Then I repeated the runs:

	result
isolated project, run 1	pass
isolated project, run 2	fail
isolated project, run 3	fail
full core suite, run 1	fail
full core suite, run 2	pass — 44 files, 516 tests, git.push.test.ts green in 1865 ms

The failure appears in both configurations and in neither reliably. My hypothesis was wrong, and the change's own honesty note under task 4.3 was right: a residual rate that no vitest configuration controls, on an environment its operating instructions describe as not a dedicated machine.

Recording that here because I nearly reported a correction the evidence does not support, and the only reason I did not is that I repeated the runs.

Also in this PR

• agentic-harness-documentation archived, complete.
• The two task notes that had recorded these failures as "co-load timeout flakiness" under load-sensitive-test-timeouts are corrected. Those notes live inside archived changes; the corrections name the earlier workaround and what it actually was, rather than silently reading as though they had always said so — which keeps the archive a record rather than a rewrite.

Test plan

• packages/core full suite — 44 files / 516 tests green (second run; see the table for the first)
• Isolated project runs recorded above
• npm run lint:english — passed
• CI green

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