docs(isaac): document a single Isaac Sim 6.0 install path (closes #108)

[yinsong1986]

What changed

Picks one supported Isaac Sim 6.0 install path and makes every surface agree on it, resolving the contradiction reported in #108.

The repo's prose (README, docs/index.md, docs/getting-started/installation.md, docs/troubleshooting.md, docs/backends/isaac.md, and strands_robots_sim/isaac/_install.py) all stated Isaac Sim is not on PyPI and must be installed out-of-band — but the [isaac] extra pinned isaacsim>=6.0 (and isaaclab>=3.0,<4.0), i.e. a PyPI install path the prose says isn't supported. Following that pin also produced a broken environment: NVIDIA's isaacsim[all] metapackage pulls the isaacsim-* packages but not the isaacsim-extscache-* packages, so SimulationApp aborts at boot with:

[Error] [omni.ext.plugin] Failed to resolve extension dependencies. ...
[Error] SimulationApp._start_app: Application failed to start.

and create_world() never starts.

This PR commits to Option A (the issue's recommended path): Isaac Sim is provisioned out-of-band via the Omniverse Launcher / Isaac Lab / NGC Docker image (each ships a complete, bootable Kit), and the pip extra carries only the genuinely pip-installable helper.

Diff (4 files, +117 / −45)

• pyproject.toml — drop isaacsim>=6.0 and isaaclab>=3.0,<4.0 from the [isaac] extra; keep usd-core only. Comment block rewritten to explain the two reasons (internal consistency + the incomplete-metapackage boot failure).
• docs/getting-started/installation.md — fix Step 2 prose (the [isaac] extra does not install Isaac Sim; it installs usd-core + strands-robots), and add a "Step 3 — verify your install boots" end-to-end snippet (create_world → add_robot → add_object → add_camera → step → render → destroy) that surfaces the extension-resolution failure if the install is incomplete.
• docs/index.md — clarify the [isaac] extra installs only usd-core + strands-robots, not Isaac Sim.
• strands_robots_sim/isaac/tests/test_entrypoint.py — replace the assertion that the extra pins isaacsim/isaaclab with test_isaac_extra_pins_only_pip_installable_helpers, which asserts the extra pins only usd-core and must not pin isaacsim/isaaclab (drift guard so the contradiction can't return).

Why

Following the documented install yielded a non-bootable environment, and the docs disagreed with packaging about how Isaac Sim is meant to be installed. Option A is the lower-friction, already-mostly-documented path: the Launcher/Docker/Isaac Lab images bundle a complete Kit (interpreter + full isaacsim-extscache-* extension set), so the pip extra only needs usd-core.

Test surface

• hatch run lint — passes (black / isort / flake8, 51 files).
• hatch run test — 203 passed, 33 skipped (skips are GPU/install-gated). Includes the updated test_entrypoint.py (16 passed).
• No GPU on this contribution box, so the live SimulationApp boot in the new Step 3 snippet was authored from the reporter's verified sequence (create_world → … → render yielding a real (480, 640, 3) RGB + depth), not re-run here.

Acceptance criteria (#108)

• Docs describe exactly one supported Isaac Sim 6.0 install path, with no "not on PyPI" vs isaacsim>=6.0-pin contradiction. (Option A; extra no longer pins isaacsim/isaaclab.)
• Following that path yields a SimulationApp that boots and runs the quickstart. (Out-of-band Kit is complete; new Step 3 snippet verifies the boot, and Troubleshooting covers the incomplete-install failure.)

Also closes #107 (isaaclab pre-release trap)

This PR also resolves #107. The same pyproject.toml edit that drops isaacsim from [isaac] also drops isaaclab>=3.0,<4.0 — which #107 reports as independently uninstallable: the only isaaclab on the index is the pre-release 3.0.0b2, and PEP 440 excludes pre-releases from a >=3.0,<4.0 pin by default, so the whole [isaac] extra was unsatisfiable:

ERROR: Could not find a version that satisfies the requirement isaaclab<4.0,>=3.0; extra == "isaac" (from versions: 3.0.0b2)

A follow-up commit on this branch records the #107-specific root cause so it can't drift back in:

• pyproject.toml — the [isaac] comment now explains the pre-release/PEP-440 trap and notes that the fleet-RL path installs Isaac Lab from source (./isaaclab.sh -i, already documented in docs/getting-started/installation.md → Step 1 → Isaac Lab), matching [isaac] extra is uninstallable: isaaclab>=3.0,<4.0 only has pre-release 3.0.0b2 on the index #107's recommended resolution (the core quickstart needs only Isaac Sim; Isaac Lab is fleet-RL only).
• test_entrypoint.py — the isaaclab not in [isaac] drift-guard assertion message now cites [isaac] extra is uninstallable: isaaclab>=3.0,<4.0 only has pre-release 3.0.0b2 on the index #107 and the pre-release reason.

Acceptance criteria (#107)

• pip install 'strands-robots-sim[isaac]' resolves cleanly on Python 3.12. (Extra now pins only usd-core; no isaaclab pre-release pin.)
• The docs/index.md quickstart runs with exactly what [isaac] installs. (Core SimEngine surface needs only Isaac Sim — validated by the reporter on Isaac Sim 6.0 / Python 3.12 with only isaacsim, no isaaclab; Isaac Lab is the fleet-RL path, installed out-of-band from source.)

Out of scope (follow-ups, not this PR)

• The environment note about Multiple Installable Client Drivers (ICDs) / CUDA error 101 under --/renderer/multiGpu/enabled=True, and exposing GPU/device selection via IsaacConfig. That's a runtime/config change unrelated to the install-path contradiction and deserves its own issue.

Project board

This issue is on the Strands Labs - Robots board; please move #108 and #107 to In review.