chore(devx #88): remove license header injection + git pre-commit hook

[earayu]

Summary

Per @earayu2 msg=d02d70dd: agents are blocked on every commit by the addlicense git pre-commit hook (it injects Apache headers and aborts the commit asking to re-commit). The project no longer needs forced license header injection at this stage; this PR removes the friction so agents can commit smoothly.

Task #88 (canonical DevX hygiene lane per architect msg=c4c6bd09 / msg=967b3737); #87 closed as duplicate.

What is removed

File	Reason
`scripts/hooks/pre-commit`	The actual blocker — runs `make add-license` which modifies files mid-commit and forces a redo.
`scripts/hooks/README.md` + `scripts/hooks/README-zh.md`	Hook documentation (no longer relevant).
`scripts/install-hooks.sh`	Helper that copied `scripts/hooks/*` into `.git/hooks/`.
`Makefile` targets: `add-license` / `check-license` / `install-addlicense` / `install-hooks`	License binary + hook installer no longer needed; the `./downloads/addlicense` download path is also gone.
`Makefile` `env-dev` deps	No longer depends on `install-addlicense` / `install-hooks`; new clones get a clean dev environment with no license tooling.
`docs/zh-CN/development/development-guide.md` lines 67–69	Removed bullets that described the now-gone hooks + addlicense steps.

What is kept (per scope discipline)

• `make lint` / `make format` (ruff check + format) — still the canonical hygiene gate, runs in CI `lint-and-unit`.
• Existing license headers in source files — not stripped, since bulk-removing them would be a huge unrelated diff (`grep` shows ~hundreds of files). Future authors are simply no longer forced to add new headers; existing ones decay naturally.
• Unit / e2e test gates — untouched.
• CI workflows — never referenced `add-license` / `check-license` / pre-commit hooks (verified via `grep -rln "addlicense\|add-license\|check-license\|install-hooks\|pre-commit" .github/` returns no matches), so no CI changes needed.

Verification

• `make -n env-dev` produces clean output with only `uv sync` + dev banner; no addlicense download, no hook install.
• `grep -rn "license\|addlicense\|install-hooks\|pre-commit" Makefile docs/zh-CN/development/development-guide.md` returns no matches post-change.
• Diff: 6 files changed, 2 insertions(+), 298 deletions(-).
• Local agents that previously had the pre-commit hook installed will still have a stale copy in `.git/hooks/pre-commit` until they delete it manually; that is per-clone state, no repo action required. `make env-dev` for fresh clones produces no hook.

Boundary

Hygiene-only PR. No app code, no migration, no schema change. lint + unit gates from main remain in place; only the agent-friction tooling is removed.

Test plan

• `make -n env-dev` dry-run — clean
• `grep` audit confirms no remaining references in Makefile / docs
• CI `lint-and-unit` — should pass (only deletes / no source code touched)

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