Package restructuring: move core to internal/mycli and extract sub-packages

[apstndb]

Overview

Root directory has 97 .go files all in package main (~40K LOC). Project config (Makefile, go.mod, README, etc.) and Go source are mixed together, making navigation difficult.

Goal: Reduce root .go files to just main.go, move core code to internal/mycli/, then incrementally extract independent subsystems into focused sub-packages.

Phases

Phase 1: Move core to internal/mycli/

Move all Go sources except main.go to internal/mycli/ package. Root main.go becomes a thin wrapper calling mycli.Main().

• Root .go files: 97 → 1
• go install path unchanged (zero user impact)
• goreleaser binary name unchanged

Phase 2: Extract independent sub-packages

Extract files with zero or minimal internal dependencies into focused sub-packages under internal/mycli/:

Target package	Files	Dependencies
internal/mycli/stream/	StreamManager (~5 files)	pure stdlib + golang.org/x/term
internal/mycli/metrics/	ExecutionMetrics (~2 files)	pure stdlib
internal/mycli/filesafety/	file_safety (~2 files)	pure stdlib
internal/mycli/sysvar/	VarHandler, VarErrors (~4 files)	pure stdlib

Effect: internal/mycli/ ~96 → ~83 files + 4 focused packages

Phase 3: Extract formatters via interface boundary

Define a minimal FormatConfig struct to decouple formatters from *Result and *systemVariables direct references, then extract to sub-package.

Effect: internal/mycli/ ~83 → ~75 files

Phase 4: Statement/Session separation (future consideration)

Would require a SessionExecutor interface. Session has ~30+ methods, so interface bloat risk is high. May be better addressed through file naming conventions rather than package separation.

Non-goals

• enums/ stays at root (no change needed)
• internal/proto*/ stays as-is

[apstndb]

Status update: Phase 1-3 complete, Phase 4 evaluated

Completed

• Phase 1 (Phase 1: Move all Go sources to internal/mycli package #496): All Go sources moved to internal/mycli/, root reduced to main.go
• Phase 2 (Phase 2: Extract independent subsystems as sub-packages #497): Extracted decoder/, filesafety/, metrics/, streamio/ — sysvar/ abandoned (unexported struct fields in ~20 struct literals block extraction)
• Phase 3 (Phase 3: Extract formatters via interface boundary #498): Extracted format/ sub-package via FormatConfig interface boundary
• Bonus: util.go eliminated (functions dispersed to appropriate packages)

Actual results

• internal/mycli/: 97 → 80 files (17 files moved to sub-packages or eliminated)
• 5 sub-packages created: decoder/, filesafety/, format/, metrics/, streamio/

Phase 4 (#499)

Evaluated and kept open as future consideration. File naming conventions provide sufficient organization at 80 files. Session's ~30+ methods make interface extraction high-effort/low-value for now.

Sub-issue completion: 3/4 (Phase 4 intentionally deferred)