refactor: one file per command and per MCP tool; flatten internal/tiger

[nathanjcochran]

Pure file/package reorganization to make the repo easier to navigate and to bring it in line with how ghost is laid out. Three changes:

1. Moved everything from internal/tiger/ up into internal/. The extra level of hierarchy wasn't buying us anything.

2. Split the CLI command files so each command gets its own file in internal/cmd/, named to match the command in snake_case (tiger service create → service_create.go). service.go was 2000+ lines and db.go and mcp.go weren't far behind. Group commands keep their own file and hold the helpers shared across their subcommands. Test files follow the same layout, with package-wide test scaffolding in main_test.go.

3. Split the MCP tool files the same way — one file per tool, each laid out as input/output schemas, then a new*Tool() constructor, then the handler. Tool registration moved into server.go, so adding a tool is one new file plus one addTool line.

No behavior changes. The only code that isn't a straight move is the MCP tool registration: the inline &mcp.Tool{...} literals became new*Tool() functions, and registerServiceTools/registerDatabaseTools are now just lists of addTool calls. Everything else is byte-identical to before, verified by diffing every top-level declaration's source text against main.

CLAUDE.md and docs/development.md are updated to describe the new conventions.