fix: structure bundle agent diagnostics

[vycdev2]

Summary

• retain candidate validation diagnostics in a detailed bundle error while preserving the existing bundle_project string-error API
• render jett bundle --agent validation failures as structured TOON diagnostic rows instead of one prose error scalar
• add driver and CLI regressions for structured failures, successful-build invariants, and output preservation

Verification

• cargo fmt --check — passed
• cargo build — passed
• cargo test -q — passed
• git diff --check — passed

Risk

• Low: the original public bundle API remains compatible, human-readable errors are preserved, and only agent-mode validation failure rendering changes.

Part of #13

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