fix(gtd): preserve RFC 3339 due fidelity in agent-mode presentation

[ohdearquant]

Summary

Fixes #912. gtd.assign returns the normalized properties.due as a top-level due convenience field. Agent-mode presentation (crates/khive-runtime/src/presentation.rs) compacts every top-level ISO-like string except field names in PAYLOAD_TIMESTAMP_FIELDS, and that allowlist held only trigger_at (added for the same defect class in a prior fix). A future task's due was therefore minute-truncated (YYYY-MM-DDTHH:MM) on the way out, even though parse_due already guarantees a full RFC 3339 value with seconds and offset. The nested properties.due was unaffected since the inside_properties guard protects it separately.

Root cause: presentation layer only — not the parser (parse_due already normalizes correctly) and not storage (properties.due round-trips fine). The loss happens exclusively in present()'s Agent-mode timestamp-compaction pass, which treats an unlisted top-level ISO-like field the same as a generic timestamp.

Fix: add "due" to PAYLOAD_TIMESTAMP_FIELDS in crates/khive-runtime/src/presentation.rs, mirroring the existing trigger_at entry. Minimal, single allowlist addition; no change to parsing, storage, or the render shape for tasks without due.

Test plan

• cargo fmt --all -- --check
• cargo clippy -p khive-pack-gtd -p khive-runtime --all-targets -- -D warnings
• cargo test -p khive-pack-gtd (includes new assign_due_survives_agent_mode_presentation_round_trip: assigns a task with an offset-bearing RFC 3339 due, reads it back via gtd.tasks and gtd.next, re-applies Agent-mode presentation, asserts verbatim round-trip)
• cargo test -p khive-runtime presentation:: (includes new agent_does_not_compact_top_level_due unit test)
• cargo check --workspace

🤖 Generated with Claude Code

[ohdearquant]

APPROVE

Findings: 0 Blocker, 0 High, 0 Medium, 0 Low

Reviewed PR #956 (68f21b8a) against main and issue #912.

Looks Right

• The PR's root-cause claim is supported by source: render_task copies properties.due to the top-level response at crates/khive-pack-gtd/src/handlers.rs:392-412, while parse_due already returns canonical RFC 3339 at crates/khive-pack-gtd/src/handlers.rs:421-440. Before this patch, the Agent transformer compacted every ISO-like top-level string that was neither under properties nor a named payload timestamp (crates/khive-runtime/src/presentation.rs:553-559). The MCP response seam calls that exact transformer at crates/khive-mcp/src/server.rs:1420-1433.
• Adding due to the narrowly scoped payload-timestamp set at crates/khive-runtime/src/presentation.rs:389-392 is the correct presentation-layer-only fix. It does not alter parsing, storage, handler shapes, or ADR-078's output-format seam.
• Tasks without a due date keep their previous Agent response: the existing null branch drops non-lifecycle null fields before the payload-timestamp string check (crates/khive-runtime/src/presentation.rs:528-558). Other timestamp compaction remains scoped and covered by agent_still_compacts_other_top_level_timestamps_alongside_trigger_at at crates/khive-runtime/src/presentation.rs:880-891.
• The new regression uses an RFC 3339 input with a -04:00 offset and exercises gtd.assign, gtd.tasks, and gtd.next, applying PresentationMode::Agent to each result and asserting the canonical due string survives verbatim (crates/khive-pack-gtd/tests/assign.rs:165-211). The dedicated presentation test also covers the bare top-level field (crates/khive-runtime/src/presentation.rs:894-903).
• The added Rust comments contain no issue-tracker marker; git diff --check is clean.

Commands Run

• CARGO_TARGET_DIR="$HOME/.cache/shared-cargo-target/khive" cargo test -p khive-pack-gtd (from crates/): passed, including assign_due_survives_agent_mode_presentation_round_trip.
• CARGO_TARGET_DIR="$HOME/.cache/shared-cargo-target/khive" cargo clippy --workspace --all-targets -- -D warnings (from crates/): passed.
• CARGO_TARGET_DIR="$HOME/.cache/shared-cargo-target/khive" cargo fmt --all -- --check (from crates/): passed.
• git diff --check origin/main...HEAD: passed.

What I Did Not Check

• No full workspace test suite or live MCP-daemon smoke run; the changed GTD package tests and required workspace static gates were run.

Re-Review Guidance

• No re-review needed.

Domain utility: SKIPPED — no separate domain corpus was available or needed for this narrowly scoped presentation-layer review.