[v0.91.8][WP-20][csdlc-v2][tests] Initialize Gate 2 temp roots before terminal recovery

[danielbaustin]

Problem

The full cargo test --locked run while validating #5527 reports 26 failures in csdlc-v2/tests/gate2.rs. Each failing fixture reaches store initialization in a temporary root that is not a Git repository, then terminal-recovery setup invokes:

git rev-parse --path-format=absolute --git-common-dir

The command fails with fatal: not a git repository. This prevents the Gate 2 suite from reaching the behavior each test is intended to exercise.

Evidence

• Reproduced from the [v0.91.8][csdlc-v2][closeout] Normalize SOR artifact refs during terminal reconciliation #5527 issue worktree with cargo test --locked.
• Failure count: 26 Gate 2 tests.
• The focused Gate 7 lifecycle suite passes (12/12), including the new receipt-bound terminal SOR repair regression.
• Strict all-target Clippy and formatting checks also pass.

Required outcome

Choose the architecture-correct repair:

1. initialize Gate 2 fixture roots as minimal Git repositories before constructing the store, or
2. defer Git-common-dir terminal receipt discovery when initializing a nonterminal store that does not require terminal recovery.

Do not weaken the terminal receipt/common-directory invariants for real repositories.

Acceptance criteria

• All Gate 2 tests reach their intended assertions and pass.
• cargo test --locked passes for csdlc-v2.
• Terminal receipt recovery still resolves the real repository common directory and remains fail-closed.
• Add or retain a regression proving non-Git fixture behavior explicitly.
• No AWS dependency or validation.

Scope

This was discovered during #5527 validation but is not caused by the bounded terminal SOR artifact-reference repair. Track and repair it independently.

v0.91.8 Routing Correction

• Parent track: WP-20 remediation, umbrella [v0.91.8][WP-20] Remediate review findings and run release preflight #5363.
• This is an independently owned C-SDLC tooling defect, not a WP-14A [v0.91.8][WP-14A] Integrated platform acceptance and deployment #5384 platform-acceptance child.