docs: README, AGENTS.md ADR renumbering, restore readme field

- Create README.md with quick-start examples for producer, consumer,
  channels, and from_call patterns
- Update AGENTS.md Architecture Context: replace all alknet-source ADR
  references (064, 071, 093, etc.) with alkcall ADR numbers (001..047)
- Update AGENTS.md convention references to use alkcall ADR numbers
- Restore readme = "README.md" in Cargo.toml

Verification:
- 542 tests, 0 failed
- cargo clippy --all-targets -- -D warnings: clean
- cargo fmt --check: clean
- cargo doc --no-deps: clean (0 warnings)
- cargo publish --dry-run --allow-dirty: succeeds

Cargo.toml

@@ -5,6 +5,7 @@ edition = "2021"
 rust-version = "1.85"
 license = "MIT OR Apache-2.0"
 description = "Call + channels RPC: structured JSON operations, streaming subscriptions, service discovery, and N-channel multiplexing over one transport stream"
+readme = "README.md"
 repository = "https://git.alk.dev/alkdev/alkcall"
 keywords = ["rpc", "json-rpc", "multiplexing", "wire-format", "alpn"]
 categories = ["network-programming", "asynchronous", "encoding"]