docs: add CAVE integration spike (non-invasive contract)

[akgohain]

Motivation

• Provide a documentation-first spike that defines a CAVE-compatible data-flow and contract for future integration without changing current runtime behavior.
• Capture expected inputs/outputs, mapping from PyTC artifacts to CAVE concepts, and a non-invasive rollout plan to guide incremental implementation.

Description

• Add docs/integrations/cave-integration-spike.md which documents target use cases, expected inputs/outputs, artifact→CAVE mappings, design constraints, TODOs for auth/network/deploy, and actionable next steps.
• The document prescribes an explicit opt-in adapter contract (e.g. prepare_cave_payload, validate_cave_payload, publish_to_cave) and requires adapter imports to be side-effect free and inert by default.
• No runtime code or endpoint behavior was changed in this PR and no adapter scaffolding or network/auth code was added.

Testing

• Verified repository state with git status --short and git branch --show-current which succeeded.
• Committed the new file with git commit -m "docs: add non-invasive CAVE integration spike plan" which succeeded.
• No automated pytest runs were executed because no runtime code was added; the adapter contract tests will be added in a future phase and should be run with uv run pytest -q tests/test_cave_adapter_contract.py when scaffolding is present.

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Codex Task

[akgohain]

Consolidated into draft PR #165 (feat/codex-wave-integration). Closing this source draft to keep review in one place.