refactor: decompose god files, add DI constructors, migrate to slog

[jongio]

Summary

Refactoring PR addressing 7 issues from the backlog. Decomposes god files, introduces DI patterns, extracts handler boilerplate, clarifies naming, and migrates legacy logging.

Changes

Issue #233 - Split logs.go god file

• Before: logs.go at 1906 lines - a single file doing parsing, formatting, streaming, and filtering
• After: 808 lines in logs.go (types, constructors, command, execute, collect) + 4 focused modules:
  • logs_parsing.go (151 lines) - file reading, log line parsing
  • logs_formatting.go (152 lines) - text/JSON display output
  • logs_streaming.go (538 lines) - all follow* streaming methods
  • logs_filtering.go (307 lines) - filter logic, level parsing, validation

Issue #234 - Decompose checker.go

• Before: checker.go at 1077 lines mixing HTTP checks, command checks, process checks, and orchestration
• After: 394 lines in checker.go (struct, CheckService, performServiceCheck) + 3 strategy modules:
  • checker_http.go (253 lines) - HTTP health check strategy
  • checker_command.go (134 lines) - command/shell health check strategy
  • checker_process.go (305 lines) - process checks, port checks, error suggestions

Issue #236 - DI for global registries

• Added wellknown.NewServiceRegistry() for dependency injection (instead of global map)
• Added service.NewOperationManager(timeout) alongside existing singleton
• Backward compatibility preserved - existing code unchanged

Issue #191 - Clarify executor naming

• Renamed service/executor.go to service/service_process.go
• Added comprehensive doc comments explaining the distinction from internal/executor
• Added service/doc.go documenting the package's internal domains

Issue #185 - Extract MCP handler boilerplate

• Created mcp_helpers.go with handleSingleServiceOp() helper
• Refactored handleStartService and handleRestartService (from 23 lines each to 4 lines)
• Pattern demonstrated for remaining handlers to adopt incrementally

Issue #201 - Migrate zerolog to slog

• Converted all zerolog calls in checker.go, checker_http.go, and monitor.go to log/slog
• Rewrote InitializeLogging() to use slog handlers
• Pattern: log.Info().Str("k","v").Msg("m") -> slog.Info("m", "k", "v")

Issue #190 - Service package documentation (partial)

• Added service/doc.go documenting the package's 5 internal domains
• Full sub-package extraction deferred (43 importers make it too invasive for one PR)

Verification

• go build ./... passes clean
• No behavioral changes - all refactoring is structural only
• All functions maintain their original signatures and semantics

Fixes #233, #234, #236, #191, #185, #201
Partially addresses #190