docs: surface observability RFCs and scope

[dinpd]

Closes #63

Summary

• broaden the repository opening from runtime guardrails alone to runtime control, durable assurance, and observability
• distinguish implemented intent-evidence/final-evaluation capabilities from the planned two-plane OpenTelemetry and assurance architecture
• present both community RFCs with their feedback issues
• link the public Intent Observability & Assurance execution project
• align the audience, documentation entry points, and roadmap language
• update the repository About description, homepage, and topics as the companion public-metadata change

User and contributor impact

Repository visitors can now discover the action-boundary evidence RFC, outcome-observability RFC, feedback threads, and milestone-scoped execution plan directly from the landing page. The wording preserves AgentPass's runtime-gate identity and does not represent either RFC as an adopted standard.

Acceptance evidence

• git diff --check — passed
• README relative-link validation — 33 unique relative targets checked, 0 missing
• required-reference validation — both RFC documents, issues Community proposal: Agent Action Boundary Evidence model #43 and RFC: Agent Outcome Observability and Assurance Metrics #58, and the public project are present
• public project URL — HTTP 200
• repository About verification — new description/homepage plus additive agent-observability, intent-assurance, and opentelemetry topics are live
• final diff — README-only

Security and review

• no executable behavior, dependencies, permissions, or data handling changed
• focused credential-pattern scan of the diff returned no matches
• manual diff review found no secrets or trust-boundary behavior changes
• no repository Markdown linter or dedicated documentation security scanner is installed; validation used structural/link checks and manual review

About metadata

The repository About metadata is not represented in the Git diff. It was updated through GitHub as part of #63 and verified after the change.