Add Windows contributor quick start

[coneilen]

Summary

Adds a concise Windows contributor quick start for the pinned, checkout-owned
build/run/stop workflow introduced by #585, with measured timings and explicit
evidence limits.

Fixes #552.

Changes

• document prerequisites, exact pinned bootstrap/environment commands, and measured reference durations
• document the safe default sandbox build/run/stop flow, artifacts, status markers, onboarding/daemon ordering, explicit -RealProfile opt-in, and current failure remedies
• link the quick start from the root README and AGENTS.md

Test plan

RED: pwsh -NoProfile -File Tools\windows\Tests\ContributorQuickStart.Tests.ps1 -> Windows README is missing the contributor quick-start heading
GREEN: pwsh -NoProfile -File Tools\windows\Tests\ContributorQuickStart.Tests.ps1 -> PASS with 3 documentation cases executed
REGRESSION: pwsh -NoProfile -File Tools\windows\Tests\Dev.Tests.ps1 -> PASS with 4 behavioral cases executed

Additional checks:

• focused documentation contract was temporary because this layer owns only the three documentation files; its RED and GREEN logs are preserved
• PowerShell link/command/parser contract -> PASS links=2 commands=7 parsedFences=3
• git diff 7d30ee2e760f56f55fcca0d11120165f29621cd0...2c2157c952397a4e8c2388b51ee674706186e105 --check -> passed
• exact stack ancestry -> current main 6010b280c21dd56c72916e248e7bf534e241851a, lower Add Windows quick-start build and run tooling #585 7d30ee2e760f56f55fcca0d11120165f29621cd0, upper Add Windows contributor quick start #594 2c2157c952397a4e8c2388b51ee674706186e105
• Tools\windows\bootstrap.ps1 -ToolRoot .\.graphcode-tools -ProviderRoot .\.graphcode-tools\providers -> passed in 2064.1 seconds with exact Zig 0.15.2/0.16.0 and provider pins
• pwsh -NoProfile -File Tools\windows\dev.ps1 -Build -> passed on the final tree in 141.2 seconds with BUILD_STATUS=BUILT, LAUNCH_STATUS=NOT_REQUESTED, VALIDATION_STATUS=NOT_RUN, and all four .build\windows\dev\bin artifact paths
• pwsh -NoProfile -File Tools\windows\dev.ps1 -Run -> passed in 1.7 seconds with run c0502662a7324b7cacbca91b0e991491; production daemon PID 14228 started before shell PID 42384, both exact checkout-owned identities were alive, and support/LOCALAPPDATA/TEMP used the reported run sandbox
• pwsh -NoProfile -File Tools\windows\dev.ps1 -Stop -> passed in 0.8 seconds; run.json was absent, captured identities were gone, zero checkout-owned daemon/shell/zmx processes remained, and the default %USERPROFILE%\.graphcode snapshot was byte-for-byte unchanged
• no native UI interaction, onboarding completion, terminal rendering, accessibility qualification, authenticated agent behavior, persistence, packaging gate, installation, or full validate.ps1 run is claimed

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test) — not run because this documentation-only Windows layer does not change the macOS product; focused Windows docs and inherited dev tests passed
• Code follows the existing style (make check) — not run because this changes Markdown only; parser/link/command checks and git diff --check passed
• I added the test/contract before the implementation and observed the intended RED failure