feat(extension): add extension config init

[RoboShyim (roboshyim)]

Summary

Draft implementation of #1253 — mirror project config init with extension config init so existing extension checkouts get a CLI-readable .shopware-extension.yml.

Usage

# Auto-detect + interactive (TTY)
shopware-cli extension config init
shopware-cli extension config init --interactive

# Non-interactive
shopware-cli extension config init --type plugin -n
shopware-cli extension config init ./my-app --type app --name "My App" --description "…" --maintainer "…" -n

# Overwrite
shopware-cli extension config init --type plugin --force -n

Behaviour

Case	Result
Plugin dir (composer.json)	Config with zip assets+composer enabled, store.type: extension
App dir (manifest.xml)	Config with assets enabled, composer disabled
Existing config	Error unless --force or interactive confirm
Bad structure	Clear error (missing manifest / composer.json)
Name / description	Written to store.meta_title.en / store.description.en
Maintainer	YAML comment only (authoritative author data stays in composer/manifest)

Layout

• internal/extension/config_init.go — domain logic + tests
• cmd/extension/extension_config.go — extension config parent
• cmd/extension/extension_config_init.go — cobra surface

Acceptance criteria

• Create .shopware-extension.yml if missing
• Interactive prompts (type, name, description, maintainer)
• Non-interactive --type app|plugin
• Existing config: refuse / --force / interactive overwrite
• Validate structure (manifest / composer.json)
• CLI-readable defaults for build/validate/zip
• Clear errors on invalid structure
• Docs page (can follow in shopware/docs)
• Tests

Test plan

• go test ./internal/extension/ -run Init
• go test ./cmd/extension/
• Manual smoke: plugin init, refuse overwrite, --force

[Codecov Comments Bot (codecov-commenter)]

Codecov Report

❌ Patch coverage is 54.12844% with 100 lines in your changes missing coverage. Please review.
✅ Project coverage is 53.64%. Comparing base (3340b47) to head (1abe9f2).

Files with missing lines	Patch %	Lines
cmd/extension/extension_config_init.go	8.79%	83 Missing ⚠️
internal/extension/config_init.go	86.40%	17 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #1254    +/-   ##
========================================
  Coverage   53.63%   53.64%            
========================================
  Files         303      306     +3     
  Lines       23325    23543   +218     
========================================
+ Hits        12511    12629   +118     
- Misses      10787    10887   +100     
  Partials       27       27            

Flag	Coverage Δ
go-test	53.64% <54.12%> (+<0.01%)	⬆️

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