docs: document set_working_directory relative path contract in runner AGENTS.md

[github-actions]

Automated AGENTS.md update triggered by commit 6da5a09.

This PR was generated by Claude Code analyzing the diff from the latest push to main and updating any stale AGENTS.md files.

Why this belongs in AGENTS.md

Path: src/prefect/runner/AGENTS.md
Proposed text: Add a **`set_working_directory` relative path contract** bullet under "Workspace Resolver Subprocess" describing: the resolver seeds `_PULL_STEP_SOURCE_CWD` (from `deployments/steps/core.py`) with the original process CWD before running pull steps; `set_working_directory` resolves relative paths against this context var; the var is cleared after each step so only the *first* `set_working_directory` sees the original CWD; subsequent calls resolve against the CWD left by preceding steps.
Non-obvious evidence: The context var is defined in `deployments/steps/core.py`, set in `runner/_workspace_resolver.py`, and consumed in `deployments/steps/pull.py`. No single file captures the full contract. The per-step clearing behavior (in the step completion callback) is particularly subtle — it changes which CWD `set_working_directory` resolves against depending on step ordering, which is invisible to anyone reading `pull.py` alone.
Cross-cutting scope: Applies to all deployments using `set_working_directory` with relative paths via the runner/workspace resolver path. Covers the "baked-image container" deployment pattern (set_working_directory: ".") as well as chained pull steps (git_clone → set_working_directory with a relative subdir).
Why not code comment/docstring: The inline comment in `_workspace_resolver.py` documents only the clearing behavior and lives in one file. The contract has two sides in two different AGENTS.md territories. A developer modifying `set_working_directory` in `deployments/steps/pull.py` would not see the runner's inline comment; a developer modifying the workspace resolver might not check `pull.py`. AGENTS.md is the only place that spans both.
Durability: The baked-image container deployment pattern is a first-class use case and will remain relevant indefinitely. The context-var mechanism is the chosen solution and unlikely to change structurally.
Counter-evidence checked: Inline comment in `_workspace_resolver.py` (lines 350-355 post-patch) documents the per-step clearing rationale; `_PULL_STEP_SOURCE_CWD` name is descriptive; existing "Local path in-place execution" bullet covers the no-pull-steps baked-image case; tests in `TestRelativePathSetWorkingDirectory` document the scenarios. None of these expose the cross-module contract at the AGENTS.md-reading level.

Changed files in triggering push

src/prefect/deployments/steps/core.py
src/prefect/deployments/steps/pull.py
src/prefect/runner/_workspace_resolver.py
tests/runner/test__workspace_resolver.py

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[chatgpt-codex-connector]

Put this contract in an AGENTS scope that covers it

This new note documents a contract shared with src/prefect/deployments/steps/core.py and pull.py, but src/prefect/runner/AGENTS.md only applies to files under src/prefect/runner/. That means future changes to set_working_directory or _PULL_STEP_SOURCE_CWD in deployments/steps will not see the instruction, which defeats the stated purpose of preserving this cross-module behavior. Please move or duplicate the contract into an AGENTS.md whose scope includes the deployments step files, such as the common src/prefect/AGENTS.md or a new scoped file under src/prefect/deployments/steps/.

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