fix(hook): align backup filename between install and uninstall to prevent data loss

[ajianaz]

What

Fixes a data loss bug in the pre-commit hook install/uninstall lifecycle.

Bug: Backup filename mismatch between install and uninstall

Operation	Backup filename (before)	Backup filename (after)
install_hook()	pre-commit.bak	pre-commit.pre-cora.bak
uninstall_hook()	searches pre-commit.cora.bak + pre-commit.pre-cora.bak	searches pre-commit.pre-cora.bak only

Scenario:

1. User has an existing pre-commit hook (e.g., husky, lefthook, custom script)
2. User runs cora hook install → original hook backed up to pre-commit.bak
3. User later runs cora hook uninstall → cora searches for pre-commit.cora.bak (not found) and pre-commit.pre-cora.bak (not found)
4. Cora deletes the hook entirely → user's original hook is lost forever

Fix

• install_hook(): changed backup filename from pre-commit.bak → pre-commit.pre-cora.bak (matches what uninstall expects)
• uninstall_hook(): removed dead pre-commit.cora.bak path (never written by anyone)

Why

This is silent data loss. Users who trusted cora hook install over their existing hooks would permanently lose their configuration on uninstall. No error, no warning — just gone.

Testing

• cargo test --bin cora — 777 passed, 0 failed (was 776, +1 new regression test)
• cargo clippy --bin cora --tests -- -D warnings — clean
• cargo fmt --all — clean
• Cora pre-commit review — No issues found

New test backup_filename_is_pre_cora_bak verifies:

1. Source does NOT reference "pre-commit.bak" (generic, was root cause)
2. Source does NOT reference "pre-commit.cora.bak" (dead code)
3. Source consistently uses "pre-commit.pre-cora.bak" for both install and uninstall