Harden A2A/ACP agent definition validation

[IvGolovach]

Summary

This lands the A2A/ACP agent-definition work in Mesh-LLM/agents, where the mesh-native agent control plane now lives.

Users get stricter local validation for agent directories and clearer CLI diagnostics before an agent definition is accepted. The PR keeps A2A/ACP-specific ownership in the agents repo and avoids adding parallel agent crates or protocol surface to mesh-llm core.

Why

This supersedes Mesh-LLM/mesh-llm#765. That PR proved the initial direction, but review correctly pointed out that the implementation belongs in Mesh-LLM/agents.

The stale design issue Mesh-LLM/mesh-llm#692 should also be moved, updated, or closed after this agents-side PR is created so future A2A/ACP work is routed to the repo that owns it.

Diff Scope

• Harden agent directory loading and validation in mesh-agents-a2a.
• Reject malformed or incomplete public Agent Cards, missing instruction files, invalid runtime/workspace combinations, empty tool/env fields, and empty supported-interface declarations.
• Improve agents validate --json reporting for runtime/policy counts and local diagnostics.
• Keep ACP runtime defaults and mesh CLI output aligned with the stricter validation model.
• Refresh the authoring skill and README guidance around the agents repo ownership boundary.

Non-Goals

• No mesh-llm core runtime change.
• No mesh gossip, protobuf, QUIC ALPN, Skippy ABI, or host-runtime protocol change.
• No agent execution-path expansion beyond validation and CLI diagnostics.
• No cross-node mesh discovery or remote invocation behavior change.

Branch Integrity

• Base repository: Mesh-LLM/agents
• Base branch: main
• Validated base: 0e2171b26f18dfe6bd4faa390e7c9badcd28e734
• Head repository: IvGolovach/agents
• Head branch: codex/a2a-acp-agent-substrate
• Head commit: 9062ac74f922a445b7bfad03d12a71fa89c118c3
• Ahead/behind: 0 behind / 1 ahead

Diff Hygiene

Changed files:

• README.md
• crates/mesh-agents-a2a/src/registry.rs
• crates/mesh-agents-acp-bridge/src/lib.rs
• crates/mesh-agents-cli/src/agents.rs
• crates/mesh-agents-cli/src/main.rs
• crates/mesh-agents-cli/src/mesh.rs
• skills/mesh-agent-authoring/SKILL.md

Proof:

• git diff --check origin/main...HEAD: PASS, no output.
• git diff --check: PASS, no output.
• git diff --cached --check: PASS, no output.

Validation

Validation tier: Tier 2 - narrow Mesh Agents validation and CLI diagnostics refresh for superseding Mesh-LLM/mesh-llm#765 in the agents repo; no mesh-llm core protocol, gossip, runtime route, or release metadata change.

• git fetch --no-tags origin main:refs/remotes/origin/main: PASS, origin/main at 0e2171b.
• git diff --check origin/main...HEAD: PASS, no output.
• git diff --check: PASS, no output.
• git diff --cached --check: PASS, no output.
• cargo fmt --all -- --check: PASS.
• cargo test -p mesh-agents-a2a --lib: PASS, 11 passed.
• cargo test -p mesh-agents-acp-bridge --lib: PASS, 12 passed, 2 ignored live ACP smokes.
• cargo test -p mesh-agents --bins: PASS, 12 passed.
• cargo check -p mesh-agents: PASS.
• rustup run stable cargo clippy -p mesh-agents-a2a -p mesh-agents-acp-bridge -p mesh-agents --all-targets -- -D warnings: PASS.
• cargo run -p mesh-agents --bin agents -- agents validate pr-review --dir examples --json: PASS, status ok with 1 enabled ACP agent.

Ledger: not applicable - not required for selected validation tier/change family.

Version: not applicable - no release/version sync required for this non-release validation/CLI change.

Not Run

• Live OpenCode/Goose/Pi ACP harness smoke: not required for selected validation tier; live ACP tests remain ignored unless a configured harness/provider is available.
• Two-node mesh discovery smoke: not required for selected validation tier; this PR only hardens local agent definition validation and CLI reporting.

Follow-Up After This PR Opens

• Comment on Add A2A and ACP agent substrate mesh-llm#765 with this PR link and close it as superseded.
• Move, update, or close Design: A2A agents over mesh mesh-llm#692 so the old design issue does not continue to attract agents work in the core repo.

Rollback Plan

Rollback: revert this PR.

DB downgrade: not applicable.
Data repair: not applicable.
Operational caveats: none known.