chore: improve development workflow and git hooks

[dreamlx]

Summary

Review and improve the current development workflow, testing pipeline, and git hooks mechanism.

Improvement Items

P0 — Immediate

• Clean up stale files in .git/hooks/ (post-commit-v4, post-commit-update-logic.sh, post-commit.v3.backup, pre-commit.bak)
• Redirect post-commit stderr to log file instead of /dev/null

P1 — Important

• Unify hook installation entry point (codeindex hooks install vs make install-hooks)
• Version-control hook-common.sh in scripts/hooks/ and copy on install

P2 — Should Do

• Add --cov-fail-under=78 coverage gate in CI (set to 78% as current baseline, target 90%)
• Change hook shebangs from #!/bin/zsh to #!/usr/bin/env bash for portability

P3 — Nice to Have

• Replace regex-based debug code detection with ruff T201/T100 rules
• Introduce mypy type checking for core modules (informational, non-blocking in CI)

Context

See workflow summary and analysis in conversation. All hooks currently use zsh-only shebang, two separate install mechanisms exist, and post-commit errors are silently swallowed.

Resolution

All items resolved in PR #37 (merged to develop).

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