rfc: profile credential storage backends (RFC 0011)

[rmanibus]

Summary

• Add RFC 0011 for profile credential storage backends.
• Add a separate implementation checklist document (outside the RFC body).
• Update the RFC index to include RFC 0011.

Scope

This PR is docs/planning only. It does not include runtime code changes.

Files

• rfcs/0011-profile-credential-storage-backends.md
• docs/implementation-checklists/0011-profile-credential-storage.md
• rfcs/README.md

Tracking

• Epic / tracking issue: RFC 0011: Epic / Tracking issue for profile credential storage backends #93
• Child implementation issues: RFC 0011: Add profile secret reference schema and env:// resolver #87 RFC 0011: Integrate secret resolver into backup/store profile resolution #92 RFC 0011: Implement macOS Keychain backend for profile secrets #88 RFC 0011: Implement Windows credential backend for profile secrets #90 RFC 0011: Implement Linux Secret Service backend for profile secrets #89 RFC 0011: Documentation and CLI UX updates for profile secret references #91
• Milestone: RFC 0011: Profile credential storage

Notes

• RFC status is currently Draft.
• Implementation is planned as incremental follow-up work via linked issues.