feat: add shellcheckExternalSources config to make --external-sources optional

[marcusquinn]

Summary

Adds a shellcheckExternalSources boolean config option (default: true for backward compatibility) that controls whether --external-sources is passed to ShellCheck. When set to false, the flag is omitted, preventing unbounded memory growth on projects with many cross-sourcing shell scripts.

Problem

The --external-sources flag is currently hardcoded in every ShellCheck invocation (server/src/shellcheck/index.ts:142). On projects with many shell scripts that cross-source each other (e.g. 100+ scripts with source-path=SCRIPTDIR in .shellcheckrc), this causes exponential AST expansion — individual shellcheck processes grow to 3-5 GB RSS and never terminate.

With multiple editor sessions or headless AI coding tools (opencode, Claude Code) each spawning their own bash-language-server, this compounds rapidly. We observed 73 concurrent shellcheck processes consuming 17.8 GB RAM on a 64 GB machine.

The existing shellcheckArguments config only appends arguments — there's no way to remove the hardcoded --external-sources.

Changes

File	Change
server/src/config.ts	Add shellcheckExternalSources boolean config (default true) and SHELLCHECK_EXTERNAL_SOURCES env var
server/src/shellcheck/index.ts	Add externalSources to LinterOptions, conditionally include --external-sources
server/src/server.ts	Pass config.shellcheckExternalSources when constructing Linter
vscode-client/package.json	Add bashIde.shellcheckExternalSources VS Code setting

Usage

VS Code setting:

{
  "bashIde.shellcheckExternalSources": false
}

Environment variable:

SHELLCHECK_EXTERNAL_SOURCES=false

LSP initialization (for non-VS Code editors):

{
  "shellcheckExternalSources": false
}

Backward Compatibility

Default is true — existing behavior is unchanged. Only users who explicitly set it to false will see the difference.

Fixes #874
Fixes #1375

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.56%. Comparing base (84ec668) to head (e46e4a6).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
server/src/shellcheck/index.ts	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1376      +/-   ##
==========================================
- Coverage   80.59%   80.56%   -0.03%     
==========================================
  Files          29       29              
  Lines        1510     1513       +3     
  Branches      373      374       +1     
==========================================
+ Hits         1217     1219       +2     
  Misses        235      235              
- Partials       58       59       +1     

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