ticket: add yaml frontmatter parsing task

.yoi/tickets/open/20260607-223233-parse-ticket-frontmatter-as-yaml/item.md

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+---
+id: 20260607-223233-parse-ticket-frontmatter-as-yaml
+slug: parse-ticket-frontmatter-as-yaml
+title: Parse Ticket frontmatter as YAML
+status: open
+kind: task
+priority: P1
+labels: [ticket, yaml, parser, bug, panel]
+workflow_state: intake
+created_at: 2026-06-07T22:32:33Z
+updated_at: 2026-06-07T22:32:33Z
+assignee: null
+legacy_ticket: null
+---
+
+## Background
+
+Ticket `item.md` frontmatter is written in YAML-like syntax and uses YAML null values such as:
+
+```yaml
+assignee: null
+legacy_ticket: null
+attention_required: null
+queued_by: null
+queued_at: null
+```
+
+However the current Ticket frontmatter parsing path treats many scalar values as raw strings. Some fields have ad-hoc `"null"` filtering, but newer nullable fields such as `attention_required` can be read as `Some("null")` instead of `None`.
+
+Observed bug:
+
+- `workspace-panel-nonblocking-transitions` has:
+
+```yaml
+workflow_state: intake
+attention_required: null
+```
+
+- Panel should derive `Clarify` from `workflow_state: intake`.
+- Instead, `attention_required` is interpreted as set and Panel derives `Edit` with an `attention_required is set` blocker.
+
+YAML `null` is valid YAML; the bug is that Ticket frontmatter is not consistently parsed as YAML/null-aware data.
+
+## Goal
+
+Parse Ticket frontmatter as YAML data, or otherwise provide a YAML-compatible typed parsing layer, so nullable fields, lists, booleans, and strings are interpreted consistently with the file format.
+
+## Requirements
+
+- Replace or wrap the current raw string frontmatter parsing for Ticket `item.md` with a YAML-aware parser.
+- YAML null forms should parse as absent/`None` for nullable fields:
+  - `null`
+  - `Null`
+  - `NULL`
+  - `~`
+  - empty value where YAML treats it as null
+- Preserve existing Ticket records and doctor compatibility.
+- Preserve current emitted frontmatter format unless intentionally changed and documented.
+- Parse list fields such as `labels` / `risk_flags` as YAML sequences when present.
+- Preserve tolerant behavior for existing simple inline list syntax, quoted strings, and old optional fields.
+- Ensure optional string fields do not treat the literal YAML null as meaningful text.
+- Avoid broad Ticket schema redesign in this ticket; focus on parsing correctness.
+- Update tests to cover YAML null/list/bool behavior and the panel `attention_required: null` regression.
+
+## Candidate affected fields
+
+At minimum verify:
+
+- `assignee`
+- `legacy_ticket`
+- `readiness`
+- `needs_preflight`
+- `risk_flags`
+- `action_required`
+- `workflow_state`
+- `attention_required`
+- `queued_by`
+- `queued_at`
+- `labels`
+
+## Acceptance criteria
+
+- A Ticket with `attention_required: null` is parsed with `attention_required == None`.
+- A Ticket with `workflow_state: intake` and `attention_required: null` derives Panel action `Clarify`, not `Edit`.
+- Existing nullable fields no longer rely on per-field ad-hoc string `"null"` filtering.
+- YAML sequence labels/risk flags parse correctly.
+- Existing Ticket fixture tests and `target/debug/yoi ticket doctor` pass.
+- Focused tests cover null variants (`null`, `~`, empty), quoted literal string values where appropriate, and regression behavior.
+- `cargo test -p ticket ... --lib`, `cargo test -p tui workspace_panel --lib`, `cargo fmt --check`, and `git diff --check` pass.

.yoi/tickets/open/20260607-223233-parse-ticket-frontmatter-as-yaml/thread.md

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+<!-- event: create author: LocalTicketBackend at: 2026-06-07T22:32:33Z -->
+
+## Created
+
+Created by LocalTicketBackend create.
+
+---