[codex] refactor confd run startup orchestration

[abtreece]

Summary

• Split cmd/confd run startup flow into focused helpers for backend config, logging, SRV discovery, observability, template config, processor selection, and supervision.
• Added unit coverage for the new helper boundaries and shutdown supervision.
• Preserved validation-mode behavior so check/validate exits still avoid SRV/backend work.

Fixes #602

Testing

• go test ./cmd/confd
• go test ./pkg/service
• go test ./...
• go vet ./cmd/confd
• git diff --check

Note: make lint / golangci-lint run ./... failed locally before analysis with: context loading failed: no go files to analyze.

[codecov]

Codecov Report

❌ Patch coverage is 31.25000% with 77 lines in your changes missing coverage. Please review.
✅ Project coverage is 71.86%. Comparing base (9837764) to head (6d9f2c5).

Files with missing lines	Patch %	Lines
cmd/confd/cli.go	31.25%	73 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
+ Coverage   70.96%   71.86%   +0.90%     
==========================================
  Files          53       53              
  Lines        5682     5711      +29     
==========================================
+ Hits         4032     4104      +72     
+ Misses       1422     1375      -47     
- Partials      228      232       +4     

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