fix: toml-relative agent spec resolves against toml dir + clean TypeError for wrong-type graph on default path (gh #116, #117)

[dkedar7]

Ships 0.6.30, fixing two cli-local bugs.

#116 — relative agent.spec in langstage.toml resolved against cwd, not the toml's dir

Config discovery walks UP ancestor dirs to find langstage.toml, but a relative agent.spec in it (what init scaffolds: spec = "my_agent.py:graph") was resolved against the process cwd. So running from a subdirectory crashed Agent file not found even though --show-config reported the spec resolved.

Fix: a relative file spec whose source is a discovered langstage.toml now rebases onto that toml's own directory (the project root) before loading — the same way a path in pyproject.toml / tsconfig.json resolves relative to the config file. A spec from -a / LANGSTAGE_AGENT_SPEC keeps its cwd-relative base (#30 — "the file is where the user typed it"); only toml-sourced relative specs change base. The path:attr split is factored into _split_py_file_spec (Windows drive-letter safe — C:\x.py:graph) and reused by both _absolutize_file_spec (cwd) and the new _rebase_toml_file_spec (toml dir); config.toml_dir_for finds the merged file that actually defines the key.

#117 — wrong-type agent leaked AttributeError: '...' object has no attribute 'checkpointer' on the DEFAULT path

On the default persist-on path, a wrong-type agent object (graph = None, a dict, an int) crashed with a cryptic AttributeError because the durable AsyncSqliteSaver was assigned to graph.checkpointer before build_agent validated the graph. --verify and --no-persist already showed the clean message.

Fix: the durable saver assignment is guarded exactly like the non-durable _ensure_checkpointer (try/except), so a wrong-type object falls through to build_agent's actionable TypeError: build_agent expected a compiled LangGraph graph (CompiledStateGraph) ... but got NoneType. — matching --verify / --no-persist. Valid graphs are unaffected.

Tests

• test_spec_resolution.py: init-scaffold runs from a subdir (spec resolved against the toml dir); -a stays cwd-relative alongside a langstage.toml (control).
• test_wrong_type_graph.py: graph = None on the default config shows the clean TypeError, not the checkpointer leak, and exits non-zero.
• Full suite: 177 passed. ruff check + ruff format --check clean.

Closes #116, closes #117.

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