chore(deps): bump agentfield floor to >=0.1.77

[AbirAbbas]

Summary

• Bumps the agentfield floor in pyproject.toml from >=0.1.55 to >=0.1.77 to pick up the cooperative cancellation feature shipped in agentfield#542.
• Without this floor, pr-af still serves reviews on Railway but doesn't honor cancel-tree — a stuck pr-af run can be marked cancelled in the control plane, but the in-flight phase (intake / anatomy / dimension generation / reviewer parallel calls) keeps running and burns LLM spend until natural completion.

Test plan

• pip install -e . resolves to agentfield 0.1.77 or later.
• Deploy to Railway, kick off a 500-line PR review, hit cancel-tree from the control-plane UI mid-anatomy, confirm pr-af short-circuits within seconds (CancelledError surfaces in the awaited Anthropic / OpenRouter call) rather than completing all phases.

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