fix: lang install bash-3.2 crash + mirror block to AGENTS.md

[zywkloo]

Summary

• Crash fix: wtc lang install with no extra positional args crashed on macOS bash 3.2 (remaining[@]: unbound variable) due to empty-array expansion under set -u. Fixed with the bash-3.2-safe idiom ${arr[@]+"${arr[@]}"}.
• AGENTS.md support: lang install/remove now mirrors the block to AGENTS.md when it exists, matching how wtcraft patch/unpatch treats agent files. If neither CLAUDE.md nor AGENTS.md exists, exits 1 and points user at wtcraft patch.
• Test suite fix: replaced bash-4-only mapfile with while IFS= read -r in tests/unit_awk.sh — the suite was silently bailing on macOS bash 3.2 before hitting any real tests.
• Regression test: new tests/e2e_lang.sh covers all four cases (CLAUDE.md-only, AGENTS.md-only, both, neither) and would have caught the original crash automatically.

Root cause

macOS ships bash 3.2 (frozen since 2007 for GPLv3 reasons). In bash 3.2, "${empty_array[@]}" under set -u is treated as an unbound variable and errors. In bash 4.4+ it quietly produces zero args. CI runs on Ubuntu (bash 5) so this was green in CI and crashing on every Mac.

Test plan

• bash tests/e2e_lang.sh — all 5 cases pass
• bash tests/unit_awk.sh — passes on macOS bash 3.2 (no more mapfile crash)
• wtc lang install in a repo with only CLAUDE.md — patches CLAUDE.md, no AGENTS.md created
• wtc lang install in a repo with both files — patches both
• wtc lang install in an empty git repo — exits 1, mentions wtcraft patch