docs: escrow relay API, ADRs, README and testnet e2e (#96)

[moises-cisneros]

Refs #96

Part of the chain that replaces #119, split after its review. Tracker: #122.

Summary

Documentation of the relay and the testnet end to end script. Run on Serverpod 4.0.4: create_job 89293b2b…3c9a (ledger 5082571) and fund d255f14a…3b18 (ledger 5082572).

Acceptance criteria

The chain as a whole covers #96; this PR carries its slice (see Scope below).

• The app can create and fund a hire on testnet using only prepare… and submitEscrowCall (proven in the docs PR)
• A signed envelope that differs from its preparation in any field is rejected
• A caller can never prepare or submit for a hire that is not theirs
• A submit… call never throws for chain outcomes
• Each preparation has at most one ChainSubmission

Verification evidence

Docs only. The e2e script is documented in puls3_server/README.md under "End-to-end proof on testnet".

Notes for reviewers

• Review against the parent branch (feat/96-escrow-relay-10-endpoint), not main.
• Generated code, migrations and recorded fixtures are excluded from the authored count.

Chain Context

Field	Value
Chain	Escrow relay prepare and submit endpoints (#96)
Tracker PR	#122
Position	11 of 12
Base	feat/96-escrow-relay-10-endpoint
Depends on	#132
Follow-up	#134
Review budget	378 authored (378 total) / 400
Starts at	feat/96-escrow-relay-10-endpoint
Ends with	api.md, ADR-0001 and ADR-0003, the server README and tool/e2e_relay_testnet.dart.

Chain Overview

main
 └── #122 Tracker (draft, no merge)
      └── #123 protocol
           └── #124 xdr-encoder
                └── #125 stellar-codec
                     └── #126 rpc-client
                          └── #127 preparation-store
                               └── #128 hire-lifecycle
                                    └── #129 prepare
                                         └── #130 submit
                                              └── #131 hire-service
                                                   └── #132 endpoint
                                                        └── 📍 #133 docs
                                                             └── #134 openspec

Scope

• Includes: api.md, ADR-0001 and ADR-0003, the server README and tool/e2e_relay_testnet.dart.
• Excludes: The OpenSpec archive.

Autonomy

• CI is expected to pass for this PR branch
• This PR has one deliverable scope
• This PR can be rolled back without unrelated changes
• Tests, docs, or manual verification cover this unit

[moises-cisneros]

Folded into #134 to keep the chain at 4 PRs. The commits are the same, just grouped; this branch stays in the history of #134.

[TOMOKI977]

Approved. This slice is part of the code I reviewed in full on #119 at b0a4adc (all four lenses; see the review there). I checked that the tip of the chain matches b0a4adc except for the CI fix in .env.example and .gitleaks.toml (and main's #112). The follow-ups listed on #119 apply to the chain as a whole.