Document project substring filter tradeoff

[upsetbit]

Closes #101.

Summary

• document that --project / ProjectMatch is a convenience substring filter and can scan project columns
• clarify that exact project path, remote, and marker filters stay indexable and should be used when identity is known
• add code comments at the local and server query builders so future callers choose the exact fields when possible

Local validation

• go test ./internal/store ./internal/server/handlers
• devbox run -- just lint-md
• devbox run -- just lint
• just test-race

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