chore(release): prepare v3.2.0

[ZhiXiao-Lin]

Summary

Bumps Rust core, Node SDK, and Python SDK to 3.2.0 and documents the subagent task tracker / progress / cancel work that landed across #35, #41, #37, #38, #39, #40.

Changes

• Version: 5 places synced to 3.2.0 (core/sdk Cargo.toml × 3, sdk/node/package.json, sdk/python/pyproject.toml) plus Node lockfiles and Cargo.lock.
• `SubagentStatus` is now `#[non_exhaustive]` so future variants don't break exhaustive matchers. The `Cancelled` variant added in this release still counts as a one-time minor breaking change for callers matching exhaustively today — called out in the CHANGELOG.
• CHANGELOG.md — new `[3.2.0]` section covering tracker query API, SubagentProgress emission, Cancelled + cancel_subagent_task, Node + Python SDK exposure, the regen-stable .d.ts split, and the TaskExecutor / register_task_with_mcp signature additions.
• README — new "What's new in 3.2" block and a Delegation table that lays out the observe + cancel API across Rust / Node / Python.

Preflight done locally

• `cargo fmt --all --check` clean
• `git diff --check` clean
• `scripts/check_release_versions.sh` reports consistent at 3.2.0
• `cargo test -p a3s-code-core --lib` — 1661 passed / 0 failed
• `cargo test -p a3s-code-core --tests` — all binaries green
• `(cd sdk/node && npm run build:debug && npm test && npm run test:types)` clean
• `A3S_CONFIG_FILE=.a3s/config.acl cargo test --test test_task_permission_inheritance -- --ignored` — 2 passed (validates the modified TaskExecutor path under real LLM)
• `A3S_CONFIG_FILE=.a3s/config.acl cargo test --test test_auto_delegation_real_parallel -- --ignored` — 2 passed (validates auto-delegation + builtin subagent execution under real LLM)

What ships after this PR merges + `v3.2.0` tag is pushed

`release.yml` triggers on `v*` tag push and:

• runs CI gates (fmt, clippy, version check)
• `cargo publish` → crates.io
• `publish-node.yml` → npm
• `publish-python.yml` → PyPI
• creates a GitHub Release

This commit itself is just the prep — tagging happens separately after merge.

Breaking changes (also in CHANGELOG)

• `TaskExecutor::execute` / `execute_parallel` / `execute_background` gained a `parent_session_id` parameter — direct callers must pass `None` (or the parent session id) to keep current behavior.
• `register_task_with_mcp` gained a trailing `subagent_tracker: Option<Arc>` parameter — pass `None` to opt out.
• `SubagentStatus` gained a `Cancelled` variant. Now `#[non_exhaustive]`, so future additions are non-breaking.