Fix shell completion value metadata

[Widthdom]

Summary

• Separate display metavariables from canonical completion value kinds and finite domains in CliFlagSchema.
• Generate Bash, Zsh, Fish, and PowerShell value completions from the shared metadata, including dynamic language/kind aliases, command-specific domains, contextual path completion, and the mixed --open-issues github + file behavior.
• Add cross-shell regression contracts, bilingual user/developer documentation, and a bilingual changelog fragment.

Root cause

Pipe-separated help placeholders were treated as exhaustive enum domains. Mixed and free-form metavariables such as <name|path> and <path|github|github:owner/name> therefore leaked into generated shell completion candidates as literal values.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore — passed with 0 warnings and 0 errors.
• Focused CliFlagSchemaTests|ConsoleUiTests on net8.0 and net9.0 — 301 passed on each target framework.
• Full net8.0 suite on latest origin/main — 11,008 passed, 7 skipped, 0 failed.
• Full net9.0 suite on latest origin/main — 10,533 passed, 420 skipped; one unrelated high-load MCP audit test failed only during concurrent dual-TFM execution and passed the immediate bounded isolated retry (1/1). A prior sequential full net9.0 run also completed without failures.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore — passed.
• dotnet run --project tools/CodeIndex.Changelog -c Release --no-build -- check — validated 78 fragments.
• Generated Bash and Zsh scripts passed syntax checks; generated PowerShell parsed successfully. Fish source contracts passed (Fish is not installed locally).
• PowerShell smoke checks verified nested relative paths, mixed github + file candidates, and quoted paths containing spaces.
• Local index refreshed against the final commit; status --check --json reports index_matches_workspace: true.

Adversarial review

Ran the required Codex adversarial review for two rounds. Addressed both actionable findings: Fish now consumes command-specific value domains, and PowerShell delegates path values to its native filename completer so nested path prefixes and quoting are preserved.

Fixes #4902