v0.20.0 — subagent orchestration unified; ast_grep + rip_grep

Breaking changes

Subagent orchestration simplified to match Claude Code's model.

• Removed parallel_agent and join tools. Fan-out is now achieved by emitting multiple sub_agent tool_use blocks in a single assistant turn — the runtime already dispatches tool calls concurrently. No tasks-array wrapper needed.
• Renamed background → runInBackground on the sub_agent and agent tools, matching Claude Code's run_in_background.
• Background subagent outputs are auto-injected into the parent's next user turn as [Background Agent Task Completed] messages; no explicit wait/join call is needed.
• TaskRegistry removed. AgentContext.task_registry gone; AgentState::new no longer takes a task_registry argument.
• Net: ~1,100 lines deleted, zero behavioural regressions across the 640+ workspace tests.

Migration

• Rename `"background": true` → `"runInBackground": true` in system prompts, agent definitions, and any code constructing tool calls.
• Replace `parallel_agent` call sites with multiple `sub_agent` tool_use blocks in the same turn.
• Remove any use of `join` — background results arrive automatically.
• Drop `parallel_agent` and `join` from any `tools` allowlist or `disabled_tools` list.

New

Search tools

• Replaced the generic `grep` tool with two focused tools:
  • `RipGrep` — regex/text search over file contents (ripgrep-powered).
  • `AstGrep` — structural code search using ast-grep patterns.

LSP

• `e2e/lsp-smoke` — dockerized LSP integration harness.
• Hardening across `crates/lsp` (error, installer, manager, server, integration tests).

CI

• `cargo fmt`, clippy 1.95 compatibility, and e2e-harness timeout bumps.

Docs

• Full rewrite of `docs/tools-reference/agent-tools.md` for the single-tool model.
• Updated tool indexes, core concepts, architecture deep-dive, and the multi-agent tutorial.
• Migration guide in `docs/reference/changelog.md`.