Add exec mode for shell-less entrypoints

[vertti]

Summary

• Add -- command syntax that runs checks, then execs into the command
• Enables preflight as ENTRYPOINT in distroless/scratch images (no shell needed)
• Uses syscall.Exec on Unix to properly replace the process (correct PID 1 behavior)

Usage

# Wait for postgres, then start app (no shell required)
ENTRYPOINT ["/preflight", "tcp", "postgres:5432", "--retry", "10", "--"]
CMD ["/myapp"]

Works with any preflight command:

preflight tcp postgres:5432 --retry 10 -- ./myapp
preflight env DATABASE_URL -- ./myapp
preflight http localhost:8080/health -- ./myapp
preflight run -- ./myapp

Implementation

• New pkg/exec package with platform-specific implementations
• exec_unix.go: Uses syscall.Exec to replace current process
• exec_windows.go: Returns error (not supported)
• Main extracts -- args before Cobra parsing, execs after checks pass

Tests

• Unit tests for extractExecArgs function
• Unit tests for exec package
• Integration tests that build preflight and verify exec mode works end-to-end

[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.39%. Comparing base (d0f13c2) to head (4c20d15).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
cmd/preflight/main.go	73.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #80      +/-   ##
==========================================
- Coverage   91.49%   91.39%   -0.10%     
==========================================
  Files          45       47       +2     
  Lines        1799     1825      +26     
==========================================
+ Hits         1646     1668      +22     
- Misses        105      109       +4     
  Partials       48       48              

Files with missing lines	Coverage Δ
pkg/exec/exec.go	100.00% <100.00%> (ø)
pkg/exec/exec_unix.go	100.00% <100.00%> (ø)
cmd/preflight/main.go	62.16% <73.33%> (+7.61%)	⬆️

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