Define custodial escrow subtype

[okjodom]

Summary

• add custodial_escrow as a canonical PIP-01 subtype
• keep top-level networks as the canonical supported-network declaration
• define common custodial fields: custody_authority, release_authority, and refund_authority
• align release/refund conditions with nested release_rules.release_trigger and release_rules.refund_trigger
• add network-specific implementations guidance and a Lightning custodial invoice example

Closes #3

Verification

• git diff --check
• stale reference scan for removed MIP/Minmo/protocol-overview/.skills references

[Copilot]

Pull request overview

This PR extends the PIP-01 escrow descriptor specification by introducing a new canonical subtype, custodial_escrow, to describe operator-custodied escrow flows while preserving the existing top-level networks field as the canonical supported-network declaration.

Changes:

• Adds a new canonical subtype section: custodial_escrow, including common custodial authority fields and lifecycle guidance.
• Defines an implementations array for network-specific implementation profiles constrained to the declared networks.
• Updates the “Open Question” note to include custodial_escrow alongside lightning_hold_invoice.

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