fix(local,keychain,api): prefer keychain for secrets, disable stdio MCP by default, surface approval-record failures

[ra-co88]

What

Three defaults change in the shipped local config:

1. The keychain plugin registers before the file store — minted OAuth tokens default to the OS keychain where one is reachable.
2. dangerouslyAllowStdioMCP defaults to false — stdio MCP servers are off unless explicitly enabled.
3. Approval-record persistence failures are surfaced through the error-capture channel instead of being silently swallowed.

Why

Tokens defaulted to the plaintext file store even on platforms with a durable keychain — the weaker storage became the default by registration order, not by decision. Stdio servers spawn local processes; that is not a default a security-conscious distribution should ship. And a swallowed approval-record failure meant a pause that never recorded its approval — invisible breakage.

What changed

• describeKeychainAvailability() encodes the platform truth (reachable/unreachable/unsupported) driving the ordering; the config consults it.
• The MCP plugin already rejects stdio with a clear error when disabled — the default now takes the safe side.
• recordPendingApproval failures report through the host's error capture; execution behavior is unchanged (the pause still returns paused, not a 500).

Headless/Linux behavior is unchanged: where the keychain probe fails, the file store remains the default.

Test plan

• Availability helper platform-truth encoding (3 tests).
• Config contract: ordering + stdio default (3 tests).
• Approval-record failure surfacing (2 tests).

All green against current main.