refactor(core): propagate context and errors

[ekalinin]

Summary

• propagate context.Context from the CLI entry point through app, controller, use cases, ports, and HTTP requests
• return explicit use case errors instead of encoding failures as nil TOCs
• aggregate document errors while preserving successful output
• return a non-zero exit code and omit the footer when processing fails
• add tests for dependency failures, missing files, empty TOCs, context cancellation, and partial success

Checks

• go test -count=1 ./...
• go test -race -count=1 ./...
• GOTOOLCHAIN=go1.21.13 go test ./...
• go vet ./...
• golangci-lint run
• go mod tidy -diff

[codecov]

Codecov Report

❌ Patch coverage is 59.62733% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.13%. Comparing base (8c71213) to head (11b558c).

Files with missing lines	Patch %	Lines
internal/controller/stdin.go	0.00%	12 Missing ⚠️
cmd/gh-md-toc/main.go	33.33%	10 Missing ⚠️
internal/controller/file.go	62.96%	8 Missing and 2 partials ⚠️
internal/adapters/tocgrabber_json.go	14.28%	3 Missing and 3 partials ⚠️
internal/adapters/tocgrabber_re.go	14.28%	3 Missing and 3 partials ⚠️
internal/adapters/filechecker.go	60.00%	2 Missing and 2 partials ⚠️
internal/controller/new.go	0.00%	3 Missing ⚠️
internal/adapters/filetemper.go	33.33%	1 Missing and 1 partial ⚠️
internal/adapters/filewriter.go	33.33%	1 Missing and 1 partial ⚠️
internal/adapters/remoteposter.go	66.66%	1 Missing and 1 partial ⚠️
... and 4 more

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #61       +/-   ##
===========================================
+ Coverage   63.75%   79.13%   +15.38%     
===========================================
  Files          27       27               
  Lines         549      623       +74     
===========================================
+ Hits          350      493      +143     
+ Misses        190      100       -90     
- Partials        9       30       +21     

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