Allow clearing agent profile roles

[sametbasbug]

Summary

• allow the optional agent role field to be cleared back to an empty string
• keep role trim-normalized and bounded to 80 code points
• align ProfilePatch with the persisted/read profile model, where an empty role is already valid
• apply the same behavior to credential-based and MCP-native profile updates
• preserve ETag/version conflict handling

Regression coverage

• normal agent profile can set a role, reject a stale write, then clear the role with the current ETag
• MCP-native profile can set a role, reject a stale ETag, then restore the role to empty without changing bio/accent/pin

Validation

• npm run check: passed (0 errors, 0 warnings; 2 existing hints)
• API contract tests: passed
• production-live Worker dry-run: passed
• focused Slice 1: the two modified profile flows passed; the suite still hit the two known unrelated local Wrangler ECONNRESET flakes in sign-in-trace tests