Reserve agentminds_io extension namespace

[UzunGridera]

Summary

Reserves the agentminds_io extension namespace for AgentMinds-
specific OASF record fields. Lays the groundwork for a follow-up
PR proposing OASF Skill 1106 ("Cross-Site Pattern Confidence")
under Category 11 (Evaluation & Monitoring).

Why this is needed

AgentMinds (https://agentminds.dev) operates a cross-site agent
intelligence pool — patterns and recommendations are aggregated
across a network of registered sites and delivered back to each
connected site filtered by its specific stack.

The state describing cross-site aggregation (Beta-Bernoulli
confidence, seen counts across sites, last-verified timestamps,
drift / volatility classes) does not have a canonical OASF
equivalent today. Per AGNTCY's contribution practice, vendors
reserve a namespace before publishing records that use vendor-
specific fields.

What this PR adds

A single file schema/extensions/agentminds_io/README.md
declaring:

• Owner, contact, license
• The seven initial extension fields with types + descriptions
• A reorientation clause that defers to canonical OASF if a
  future version absorbs cross-site pool concepts

What this PR does NOT add

• No schema changes to schema/objects/
• No new skill taxonomy IDs (those will land in a follow-up PR)
• No reference code

Companion submissions (planned)

1. OASF Skill 1106 — Cross-Site Pattern Confidence under
   Category 11. Schema fragment ready; waiting for this namespace
   PR to merge.
2. MCP SEP "Cross-Site Audit Trails for Tool Invocations" —
   the same primitive at the MCP layer.

Reference implementation

The fields are already in production at AgentMinds. Reference
implementation:

• ARP fingerprint helper (Python):
  https://github.com/agentmindsdev/profile (canonical spec at
  AGENT_REPORTING_PROFILE.md §3.3)
• Empirical results: 60–80% cross-site false-positive suppression
  rate when operator marks a pattern resolved on one site.

DCO sign-off in commit message.