Teach Field default authoring

[raghubetina]

What

• teach create-full-stack-app to author and preserve all four tagged Field-default variants
• distinguish owner identity, omission, literal null, enum-key coupling, and canonical decimal strings
• extend examples and behavior evals for enum defaults, environment and decimal defaults, coordinated enum-value renames, and multi-diagnostic recovery
• pin the guidance to First Draft server baseline 944673772ef7c5bf40ff67e65d8266556ab75f08

Why

Field defaults are now retained by the bounded importer, but the Skill still described them as unsupported. Agents need enough guidance to author good values without inventing subject IDs, weakening product meaning, or mistaking structural acceptance for semantic analysis.

Verification

• sh script/check: 11 of 11 tests pass
• npm audit --audit-level=low: zero vulnerabilities
• gh skill publish --dry-run: succeeds with the existing license and tag-protection advisories
• regenerated two-diagnostic fixture verified against the real baseline importer
• fresh-agent enum/default authoring exercise minted exactly four subject IDs, minted none for the default, preserved private state, and made no push

Boundaries

This remains an experimental, structurally bounded authoring slice. It does not claim default type analysis, link resolution, Compilation, Publish, released end-to-end distribution, or complete graph import.

[raghubetina]

This slice has landed on main through #6, which integrated the complete reviewed Skill stack. This PR remains the focused review history for Field default authoring; no implementation was discarded. Closing as superseded by #6.