chore(lua): remove lua plugin

[hay-kot]

Summary

• Deletes the entire Lua plugin subsystem: runtime, modules (hive.commands, hive.http, hive.json, hive.kv, hive.logger, hive.sh, hive.ticker, hive.info), LuaPluginConfig, the hive plugin init scaffolder, integration tests, and docs.
• Drops github.com/yuin/gopher-lua from the module graph.
• Simplifies the plugin manager: CommandSet.MergePlugin and Manager.Pool() are gone; the seeding flow no longer needs the Lua re-init dance.
• 47 files, -6939 lines.

Why

The Lua plugin was an experiment in user-extensible commands that hasn't paid for its surface area. Removing it cuts a runtime, a dependency, a config block, a CLI subcommand, and ~7k lines of code+docs+tests with no migration path needed.

Phased commits

Each phase is a self-contained commit; build is intentionally broken between Phase 1 and Phase 3, then restored.

1. Delete internal/hive/plugins/lua/ (25 files, -5629)
2. Remove LuaPluginConfig + validators
3. Rewire main.go + plugin manager, drop MergePlugin/Pool() — build restored
4. Delete hive plugin init command + defaultPluginsDir()
5. Delete 6 Lua integration tests
6. Remove docs (docs/configuration/plugins.md Lua section, NeoVim comparison)
7. go mod tidy — removes gopher-lua
8. Drop a marginal orphan-key regression test that survived from an early plan revision

Full plan: .hive/plans/2026-05-11-remove-lua-plugin.md.

Test plan

• mise exec go@1.26 -- go build ./... passes
• mise exec go@1.26 -- go test ./... passes
• mise exec go@1.26 -- go test -tags integration ./test/integration/... passes
• mise run lint exits 0 (after chore(lint): remove goconst linter #331)
• mise run docs:build succeeds
• Manual: mise container — hv --help shows no plugin subcommand, TUI palette opens, hv doctor exits 0, and HIVE_CONFIG=<yaml with plugins.lua> is silently ignored