fix(git_bridge): strip inherited git-context env in run_git (kills pre-push hook race)

[joelteply]

Summary

The pre-push hook's Phase 3 (Rust tests) was reliably failing 9-of-9 code::git_bridge::tests::* whenever invoked, even though the same tests passed in isolation. Root cause is git env inheritance, not parallelism.

Root cause

git push sets GIT_DIR, GIT_PREFIX, etc. on the pre-push hook's environment. Those env vars propagate through every child — including cargo test, including the test binary, including each test's tempdir git init/git commit. So a test's git commit in its tempdir inherits GIT_DIR=/Users/.../continuum/.git, runs the parent worktree's pre-commit hook (which can't find its target script in the tempdir context), and panics.

Three different surface symptoms, all from the same upstream cause:

• "could not lock config file <bare>/.git/config: File exists"
• "Unable to create '<bare>/.git/worktrees/<x>/index.lock'"
• "<bare>/.git/hooks/pre-commit: <tmp>/src/scripts/git-precommit.sh: No such file or directory"

Fix

In run_git:

• env_remove for GIT_DIR, GIT_WORK_TREE, GIT_COMMON_DIR, GIT_INDEX_FILE, GIT_PREFIX
• Set GIT_CEILING_DIRECTORIES=workspace_root as defense-in-depth against future env vars

run_git is now context-clean: git discovers from current_dir(workspace_root) only, no parent contamination. Production callers get a small safety improvement (continuum-core's git_bridge invoked from any process won't accidentally inherit the caller's git context); test callers get reliable isolation.

Tests

Reproduces the previously-failing case by simulating the hook environment:

Command	Before	After
cargo test --lib code::git_bridge	passed	passed
GIT_DIR=<continuum>/.git cargo test --lib code::git_bridge	9 failures	9 passed
cargo test --lib (full suite, normal env)	passed	passed
cargo test --lib (full suite, via pre-push)	9 failures	passed (this push)

Why this PR matters beyond itself

The pre-push hook's Phase 3 false-positive was blocking ANY PR whose change is non-Rust (PowerShell-only, docs-only, TS-only) from passing through. Two such PRs were blocked tonight — b69f's #1005 (Carl-OOTB Windows fix #1) and my unpushed install.ps1 networking-probe (Carl-OOTB Windows fix #2). With this fix, the hook stops false-positive blocking on the inherited-env race.

Test plan

• CI green (clean-install matrix unaffected; this is Rust-only fix in continuum-core)
• Local pre-push passed (this push went through cleanly)
• Reviewers can repro: GIT_DIR=$(git rev-parse --git-dir) cargo test --lib --features metal,accelerate code::git_bridge — should pass on this branch, fail on canary

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