[Phase 13] Apply default PR base and surface config diagnostics

[cosentinode]

Scope

Apply configured default PR base with explicit/config/provider precedence, validate it through existing GitHub planning, surface safe config diagnostics in the TUI, and finish configuration examples.

Acceptance Criteria

• Precedence is explicit prompt, configured default, then provider default.
• Missing configured base fails closed with actionable guidance.
• Explicit base is never overridden.
• Invalid config visibly reports safe-default fallback without exposing contents.
• GitHub.com and Enterprise behavior match.

Verification

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace

Notes

Child of #15 and final Phase 13 slice.