feat: rename default project version from empty string to "v0"

[migmartri]

Summary

The default project version (the un-versioned bucket that workflow runs fall into when no --version is specified) is stored as an empty string "" in the database. This is referred to colloquially as "v0" in documentation and conversation, but the actual stored value causes widespread friction:

• Empty strings are opaque in APIs, CLIs, and MCP tools — every consumer must special-case "" to understand it means "the default version"
• Declarative access is broken: users cannot say --version v0 to target the default; they must omit the flag entirely
• CLI display is inconsistent: attestation status shows "none", workflow run list shows a blank column

Proposed Change

Rename the stored value from "" to "v0" everywhere:

1. Add DefaultVersionName = "v0" constant in the biz layer
2. Update Ent schema: change Default("") → Default("v0"), remove the empty-string validator exception
3. Update application code that creates/looks up the default version (workflow.go, workflowrun.go) to use the constant
4. Add backward-compat translation: server translates incoming "" → "v0" so old CLI clients keep working
5. Database migration:
   • Rename any existing user-created "v0" versions → "v0.0" (conflict avoidance)
   • Rename all "" versions → "v0"
   • ALTER TABLE project_versions ALTER COLUMN version SET DEFAULT 'v0'
6. Remove "none" fallback from CLI display — "v0" renders naturally as a real version string

Behavior After Change

Scenario	Before	After
Run without --version	Associates with "" version	Associates with "v0" version
Run with --version v0	Creates a NEW "v0" version	Finds existing "v0" version
Old client sends ""	Creates/finds "" version	Server translates to "v0"
CLI attestation status	Shows "none"	Shows "v0"
CLI workflow run list	Shows blank	Shows "v0"