(mcp) Enforce hard response budgets

[tcballard]

Implements #411. The native MCP server now treats the response budget as a hard successful-payload limit across stdio and HTTP.

What changed

• Enforce deterministic hard caps in the shared Rust serializer for summaries, deep relationship neighborhoods, outgoing targets, attention, matches, items, and artifact content.
• Return a small structured response_budget_exceeded payload when fixed fields cannot fit instead of returning an oversized success.
• Add decided-mcp --budget N for both transports with an explicit 128-character minimum; reject undersized per-call budgets for get_artifact and retrieve_grounding.
• Thread the configured budget through stdio and HTTP without changing frozen protocol/list bytes.
• Add ADR-128, update the MCP wire contract and public docs, and add deterministic/unit plus end-to-end stdio/HTTP coverage.

Validation

• cargo test --workspace
• cargo clippy -p asdecided-core -p decided-mcp --all-targets -- -D warnings
• git diff --check
• Rebased onto the merged (mcp) Restore complete and attributable audit records #424 commit; the PR now contains only the nine budget files and is ready to squash-merge once GitHub CI is green.