Fix install lifecycle and update commands

[Widthdom]

Summary

• add install.sh --uninstall [--purge-cache] for removing installed payloads and cdidx cache metadata
• add explicit update checks via cdidx --check-updates, status --check-updates, and cdidx upgrade/upgrade --check-only
• add workspace .cdidx-version pin warnings with strict failure mode
• add README/DEVELOPER_GUIDE docs and changelog fragments for each fixed issue

Validation

• dotnet build CodeIndex.sln -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -p:UseSharedCompilation=false --filter "FullyQualifiedName~ProgramRunnerTests|FullyQualifiedName~InstallScriptTests|FullyQualifiedName~CliFlagSchemaTests"
• adversarial review: No blocking/actionable issues found.
• smoke: CDIDX_DISABLE_UPDATE_CHECK=1 dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll upgrade --check-updates --json
• smoke: CDIDX_DISABLE_UPDATE_CHECK=1 dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll --check-updates --json
• smoke: CDIDX_DISABLE_UPDATE_CHECK=1 dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check-updates --json --db .cdidx/codeindex.db

Notes

• No AGENTS.md, CLAUDE.md, or AGENT_GUIDE.md changes are included in this branch diff.
• Issue comments were checked before starting; none of No self-update / upgrade subcommand - users must manually re-run install script #1744, No --check-updates / version-freshness ping - operators have no signal a newer release is available #1748, No uninstall script or documented uninstall procedure #1750, or No workspace-level binary version pinning / lock file - team-wide version skew undetectable #1752 had work-start comments.

Fixes #1744
Fixes #1748
Fixes #1750
Fixes #1752