Add Brickken Base MCP plugin

[MethosPi]

Adds a native Base MCP plugin for Brickken ERC-8004 agent identity, reputation, and agent-owned ERC-20 operations.

The plugin supports:

• Path A: Brickken prepares unsigned calldata, Base MCP submits via send_calls when x402 settlement from the Base Account succeeds.
• Path B: Brickken CLI/MCP self-submits when configured with an EOA key.
• Mainnet x402 cost disclosure, native gas requirements, and sandbox caveats.

Also registers Brickken in the Base MCP native plugins table.

Validation:

• Checked against skills/base-mcp/references/plugin-spec.md required frontmatter and canonical headings.
• Ran git diff --check.

[cb-heimdall]

🟡 Heimdall Review Status

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