chore(project)!: drop the deprecated flat environment command names

> 🤖 Generated by the Daily AI Engineer

**Problem.** #6059 (`Fixes #6057`) restructures environment management into the `project env` group and keeps the former flat names — `project add-environment`, `project list-environments`, and the `cluster add-environment` alias from #5626 — as hidden, deprecated delegates so released invocations keep working.

**Proposed direction.** After a deprecation cycle (at least one minor release with the delegates in place), remove the three delegates and their generated doc pages in a single breaking change (`chore(project)!:` → major release). Mechanical: delete the delegate constructors + their pins in project/cluster tests, regenerate docs/snapshots.

**Blocked by** #6059 merging and shipping in a release.

**Rough size.** S.

Wait for #6059 to merge and ship in a release, then inspect the project and cluster command constructors and their project/cluster tests. Remove the three deprecated delegates and their test pins, then regenerate the documentation pages and snapshots. Done means the flat project commands and cluster alias are removed together in the planned breaking release.

Written by the indexing model from the issue text.