feat(skill): one approval, then onboarding's one question; remove /tskl

[theCodeDrift]

In a real "let's use taskless" session, the user went through four turns before onboarding asked its one question: a blocked preflight, a failed settings write, a menu of topics, then "set it up". This PR cuts that down to one approval at most, followed by onboarding's question.

Skill

• On a blocked command the agent sends one message with three options. It can add the exact-invocation allow rule itself, and if its harness refuses that edit, it hands the user the change to make. The user can approve the invocation with their agent's permissions command (/permissions with Bash(<invocation>:*) in Claude Code). Or the user can approve every Taskless command in chat.
• The agent no longer asks what the user wants, either while blocked or once agent ok passes. It goes straight to the request.
• A request that names no task ("use", "try", or "set up" Taskless) goes straight to agent onboard.

/tskl removed

• The command source and the plugin's commands entry are gone, along with the installer's command writing, buildCommandStub, and writeCanonicalCommand.
• The manifest still records commands, so an upgrade deletes the tskl.md an older install wrote, from .taskless/commands/ and from each tool's commands/tskl/. Empty directories are removed. A tool's other command files are kept, and a symlinked tskl/ is unlinked rather than deleted through. New tests cover each case.
• init --json drops commandsInstalled and writtenCommands, and keeps removedCommands. The init recipe goes to topic v6.
• Existing projects' .taskless/README.md keeps one stale commands/ line until it's next regenerated. The template is fixed for new installs; a schema migration for one line of docs didn't seem worth it.

Spec and release

• The OpenSpec change one-approval-one-question is archived here. Every scenario that left the specs belonged to a requirement this change removes.
• remove-tskl-command is a minor changeset, because /tskl users have to change how they ask. The pending release is already minor.
• agent-ok-preflight is corrected in place, since it is still unreleased.