fix(ownership): fail closed on WASM default-scope fallback

[G7CNF]

Fixes #2069
Refs #2070

What changed

• WASM tool credential resolution now fails closed instead of borrowing the default scope for admin users.
• The tool-facing error now names the user and tells them to configure the missing credential with ironclaw secrets set.
• WASM channel startup no longer falls back to legacy default broadcast metadata when the owner scope has no stored value.
• Added regressions proving we only read the configured owner scope and do not probe default.

Notes

• #2070 overlaps on the same ownership boundary. The setup docs already document config.owner_id as the correct owner scope for instance-level channel secrets, so this PR keeps the runtime fix focused on the unsafe fallback behavior.

[gemini-code-assist]

Code Review

This pull request removes legacy fallback logic for broadcast metadata to ensure strict owner-scoping and hardens WASM tool credential resolution by denying default scope fallbacks. It also improves error reporting for missing credentials and introduces a mock settings store for testing. However, the mock's has_settings method is inconsistently implemented compared to other methods in the store and should be updated to correctly verify the presence of stored values.

[henrypark133]

Review: fail-closed ownership fix looks correct

I verified the two ownership-scope changes in the checked-out code and did not find a blocker.

Positives:

• WASM tool credential resolution now uses DefaultFallback::Denied, so the runtime no longer borrows default-scoped secrets when a user-specific secret is missing.
• WASM channel startup only reads owner-scoped broadcast metadata and no longer probes default.
• The new regressions cover both the tool path and the channel-startup path, which is the right caller-level coverage for this boundary.

Residual risk:

• Approval is based on code inspection plus the new targeted tests; I did not run the full suite from this review pass.