fix(docs): rewrite env README relative links in generated stubs

[Cunninger]

Fixes #1095

scripts/sync_env_docs.py inlined each env README into docs/source/environments/<slug>.md but only rewrote HTML src="assets/" image paths. Markdown links such as [example](../../examples/foo.py) stayed relative, so they resolved from the docs stub (wrong depth) on the published site.

This extends stub generation to rewrite markdown links that escape envs/<env>/ into GitHub blob/tree URLs (the convention browsergym / pelican_svg already use by hand). Extra ../ segments that walk out of the repo (e.g. git_env's ../../../examples/...) are stripped so they still map to the intended path.

--check already treats rewritten stubs as the expected content, so leftover relative links fail CI until --fix is run. Regenerated the currently affected stubs (agent_world_model, carla, git, opencode, repl).

Tests:

PYTHONPATH=src:envs uv run pytest tests/scripts/test_sync_env_docs.py -v
uv run python scripts/sync_env_docs.py --check

9 passed; --check is clean.

RFC: not required (docs generator only, no public API / core change).

---

Note

Low Risk
Docs sync tooling and generated markdown only; no runtime API or environment behavior changes.

Overview
Fixes broken example and RFC links on the published docs site when env READMEs are inlined into docs/source/environments/*.md (issue #1095). Previously only HTML assets/ image paths were absolutized; markdown links like ../../examples/foo.py stayed relative and resolved from the wrong path.

scripts/sync_env_docs.py now rewrites markdown links that point outside envs/<env>/ into GitHub blob/tree URLs (and raw for images), including normalizing over-long ../ chains (e.g. git_env’s ../../../examples/...). Stub generation calls this during generate_stub, so --check in CI expects the rewritten output.

Regenerated affected stubs (agent_world_model, carla, git, opencode, repl) and added tests/scripts/test_sync_env_docs.py for path resolution, link rewriting, and integration via generate_stub.

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