Fix excerpt aliases, guard argument handling, and progress opt-out

[Widthdom]

Summary

• Add MCP-style --start-line / --end-line aliases for query --excerpt.
• Tighten the Bash command guard so forbidden command names in GitHub arguments are allowed while real shell/script search commands remain blocked.
• Add progress animation opt-outs through --no-progress, CDIDX_DISABLE_PROGRESS=1, and PREFERS_REDUCED_MOTION.

Validation

• python3 -m unittest discover -s .agent_harness/tests -p 'test_command_guard_core.py'
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release --filter "RunExcerpt_AcceptsMcpStyleStartAndEndLineAliases|ShouldUseProgressAnimation_EnvOptOutDisablesAnimation|ShouldUseProgressAnimation_ReducedMotionDisablesAnimation|TryConsumeNoProgressFlag_RemovesGlobalFlagAndDisablesAnimation|FormatProgressLine_ProgressAnimationDisabled_UsesStaticPrefix|PrintCompletions_ReportFlagSetsMatchAcrossShells" -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false -p:EnableAotAnalyzer=false -p:EnableTrimAnalyzer=false
• Adversarial review: No blocking/actionable issues found.

Full Release build/test without disabling AOT/trim analyzers currently fails on latest origin/main with existing IL3050/IL2026 analyzer diagnostics unrelated to this branch.

Changelog

• changelog.d/unreleased/1429.fixed.md
• changelog.d/unreleased/1432.fixed.md
• changelog.d/unreleased/1630.added.md

Notes

• No target issue was excluded for an existing work-start comment.
• No additional related issues were included.
• No new issue was filed.
• AGENT.md, CLAUDE.md, and AGENT_GUIDE.md were not modified.

Fixes #1429
Fixes #1432
Fixes #1630