fix(docs): resolve broken links and legacy mdBook routes in specs and agents

[vhtgzzl]

Summary

This PR fixes broken internal links and legacy mdBook-style routes across the Base documentation site (\docs.base.org):

1. Base Agents Landing (\docs/agents/index.mdx):

   • Updates the broken /base-account\ link to the canonical Base Account overview path: /base-account/overview/what-is-base-account.

2. Protocol Specifications Overview (\docs/base-chain/specs/protocol/overview.mdx):

   • Replaces 15 legacy mdBook .html\ extensions and relative directory paths with trailing slashes across Mermaid architecture diagrams and section navigation buttons (\OptimismPortal, \BatchInbox, \DisputeGameFactory, \AggregateVerifier game, \Batcher, \Consensus, \Execution, \Proofs, \Bridging).
   • Ensures all interactive diagram nodes and buttons route properly in Mintlify without producing 404 errors.

Changes Made

• \docs/agents/index.mdx: Updated [Base Account](/base-account)\ to [Base Account](/base-account/overview/what-is-base-account)\
• \docs/base-chain/specs/protocol/overview.mdx: Updated 15 legacy links in Mermaid diagrams and section links to canonical Mintlify routes.

Verification

• Audited all internal and relative link targets across \docs/\ to ensure zero broken links.
• Verified that no .html\ artifacts or broken paths remain in the specs diagrams.
• Verified working tree status and clean commits following Conventional Commits format.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1