docs(devx): anchor the isSystem census page by symbol, not by line number (#16614)

* wip: migrate system-context.mdx anchors to path#symbol

* feat(devx): hold the system-context census page by symbol anchors, not line numbers

* chore: drop a scratch probe committed by mistake

* docs(devx): spell the census gate's own header anchors from the repo root

* fix(devx): detach the census gate's self-test from an inherited git environment

Its throwaway red-first corpus ran `git init` / `git add -A` with the ambient
environment. Under `pre-commit` — where `check-regen-pending` invokes this gate,
and where an os-regen merge lap lands — git has exported GIT_DIR, GIT_WORK_TREE
and GIT_INDEX_FILE, so the corpus was never created and the repository's own
index was written instead: 8,190 paths staged as deleted, with every self-test
case still printing ok.

The self-test now strips every GIT_* key before its first case and the corpus
builder passes the stripped environment explicitly. A regression pin injects a
bogus GIT_DIR and requires the throwaway tree to come back with its own two
files staged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Vbw3RPgdtqesx4azk9SbW8

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: claude[bot]