refactor(acp): migrate to agent-client-protocol 2.0.0

[bug-ops]

Summary

• Bumps agent-client-protocol 1.2.0 -> 2.0.0 (schema =1.4.0 -> =1.5.0) in crates/zeph-acp, per the migration plan in specs/013-acp/spec.md. Mechanical crate-major bump: the wire protocol Zeph speaks stays at V1; zero handler/transport/builder-chain source changes were needed (the three named compiler-verify points from the spec all compiled clean).
• Adds a compile-time regression guard (crates/zeph-acp/src/lib.rs) plus an independent runtime test that both hardcode the wire version, so a future schema-pin bump can't silently redefine ProtocolVersion::LATEST away from 1 without failing the build/tests.
• Adds an integration test covering the SDK's newly-accepted JSON-RPC batch requests end to end through the dispatch loop (a new input path introduced by 2.0.0 that had zero test coverage).
• Fixes specs/013-acp/spec.md sections that still asserted a stale pre-1.2.0 protocol version as current state, plus several other inaccuracies uncovered during review (the /agent.json vs /.well-known/acp.json handler/route table, cargo-deny evidence, Implementation Gap Tracker cross-references).

Process notes

This went through the full refactoring chain (developer -> parallel tester/security/impl-critic -> code review), with one consolidated fix round after the critic's significant verdict (3 blocking gaps: stale README version, unverified fork/providers test-coverage claims, untested batch-dispatch path) and the tester/security findings from the same pass. The reviewer found and fixed two more issues live with the developer during final review (a stale /agent.json example in the spec, and a wrong cargo-deny invocation/evidence). Security's audit found zero new vulnerabilities from this bump; it separately surfaced a pre-existing (not introduced by this PR) HIGH-severity permission-gate deadlock, filed as #6656.

Closes #6655
Closes #6633

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins (15073 tests, all passing)
• cargo nextest run -p zeph-acp with the full unstable-feature set (222 tests, incl. 2 new regression tests)
• Rustdoc gate (RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace ...)
• cargo test --doc --workspace ...
• cargo deny --config .github/deny.toml check (zero new advisories from the bump; same 3 pre-existing accepted advisories)
• gitleaks protect --staged --no-banner --redact
• Live ACP round-trip: initialize -> session/new -> session/prompt against a real local Ollama model, plus fork/resume/session/delete/logout/set_config_option/set_mode/block_task exercised by the zeph-acp integration suite under 2.0.0; /agent.json and /.well-known/acp.json field exposure verified against the actual handler code (only the latter carries protocol_version, confirmed against the upstream ACP Registry RFD)