feat(cli): add completion installer; standardize output mode; add 'struct init' scaffolder

[httpdss]

This PR completes more of #97 (CLI UX and discoverability).

Adds:

• struct completion install [bash|zsh|fish] (prints enablement steps)
• Standardized output mode naming across MCP to console|file (default file) with back-compat for files
• New struct init that writes a basic .struct.yaml including:
  • pre_hooks/post_hooks with echo commands
  • files with a README.md placeholder
  • folders referencing the github/workflows/run-struct workflow at ./

Tests:

• New unit tests for completion install command
• New unit tests for struct init creation and skip-if-exists behavior
• All tests passing locally

Docs:

• README, CLI reference, and completion guide updated previously
• MCP integration doc updated to reflect console|file

Refs: #97

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 65.99%. Comparing base (d7c056a) to head (08f162e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
struct_module/commands/completion.py	83.33%	5 Missing and 3 partials ⚠️
struct_module/main.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #106      +/-   ##
==========================================
+ Coverage   65.44%   65.99%   +0.54%     
==========================================
  Files          18       19       +1     
  Lines        1285     1335      +50     
  Branches      223      229       +6     
==========================================
+ Hits          841      881      +40     
- Misses        380      387       +7     
- Partials       64       67       +3     

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