Remove pipectl init command

[khanhtc1202]

What this PR does:

We don't prioritized this command any more. Also it's recommend to use MCP pipecd if users want to have support around app config generation. (ref: https://pipecd.dev/blog/2025/05/26/mcp-server-for-pipecd-docs-has-been-released/)

Why we need it:

Which issue(s) this PR fixes:

Part of #5956

Does this PR introduce a user-facing change?:

• How are users affected by this change:
• Is this breaking change:
• How to migrate (if breaking change):

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.05%. Comparing base (f39124f) to head (e197788).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5958      +/-   ##
==========================================
- Coverage   28.50%   28.05%   -0.45%     
==========================================
  Files         520      515       -5     
  Lines       56434    55822     -612     
==========================================
- Hits        16084    15662     -422     
+ Misses      39083    38910     -173     
+ Partials     1267     1250      -17     

Flag	Coverage Δ
.	23.22% <ø> (-0.59%)	⬇️
.-pkg-app-pipedv1-plugin-example	0.00% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes	66.42% <ø> (ø)
.-pkg-app-pipedv1-plugin-kubernetes_multicluster	67.51% <ø> (ø)
.-pkg-app-pipedv1-plugin-wait	35.51% <ø> (ø)
.-pkg-plugin-sdk	48.86% <ø> (ø)
.-tool-actions-gh-release	19.23% <ø> (ø)
.-tool-actions-plan-preview	25.30% <ø> (ø)
.-tool-codegen-protoc-gen-auth	0.00% <ø> (ø)

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[ffjlabo]

LGTM

[t-kikuc]

nice