Unify CLI help metadata and add conventional context controls

[Widthdom]

Summary

• Centralize command, nested-subcommand, and flag metadata used by parser validation, command help, next-step hints, and Bash/Zsh/Fish/PowerShell completions.
• Add conventional cdidx help <command> [subcommand] routing with public aliases, nested aliases, suggestions, and stable usage errors without dispatching the target command.
• Add symmetric find --context <n> support with order-independent asymmetric overrides, bounded values, literal-query safety, batch parsing, help, and completion coverage.
• Add status --explain sqlite_connection_policy metadata parity and correct the documented search/help and workspace completion contracts.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore — 0 warnings, 0 errors
• dotnet test CodeIndex.sln -c Release --no-build --no-restore
  • net9.0: 9,046 passed, 384 skipped, 0 failed
  • net8.0: 9,482 passed, 6 skipped, 0 failed
• Targeted Release tests for Generate command help and shell completions from shared command metadata #4571, Support the conventional help <command> CLI form #4575, and Add --context as the common symmetric context option for find #4621: 28 passed per target framework
• dotnet run --project tools/CodeIndex.Changelog -- check — 42 fragments validated
• Generated Bash behavior and Zsh/PowerShell completion syntax verified
• Two Codex adversarial review rounds completed; all actionable findings addressed

Documentation and changelog

Updated the bilingual English/Japanese sections in USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md, plus:

• changelog.d/unreleased/4571.fixed.md
• changelog.d/unreleased/4575.added.md
• changelog.d/unreleased/4621.added.md

Follow-up

• Release MCP teardown deadline test is timing-sensitive when isolated #4653 tracks the pre-existing timing sensitivity observed when the MCP teardown deadline test is run in isolation.

Fixes #4571
Fixes #4575
Fixes #4621