feat(projman): add sprint lifecycle state machine via milestone metadata

- New skill: sprint-lifecycle.md defines states, transitions, and check protocol
- All sprint commands now check and set lifecycle state
- States tracked in milestone description metadata (Sprint/Planning, Sprint/Executing, Sprint/Reviewing)
- Out-of-order calls produce warnings with guidance
- --force override available for all lifecycle checks
- Added Sprint/* labels to label taxonomy documentation

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

plugins/projman/skills/label-taxonomy/labels-reference.md

@@ -90,6 +90,18 @@ Repository-level labels are specific to each project.
 - `Tech/Vue` (#42b883) - Vue.js frontend framework
 - `Tech/FastAPI` (#009688) - FastAPI backend framework
 
+### Sprint Lifecycle (Milestone Metadata)
+
+These are tracked as milestone description metadata, not as Gitea issue labels. They are documented here for completeness.
+
+| Label | Description |
+|-------|-------------|
+| `Sprint/Planning` | Sprint planning in progress |
+| `Sprint/Executing` | Sprint execution in progress |
+| `Sprint/Reviewing` | Code review in progress |
+
+**Note:** Lifecycle state is stored in milestone description as `**Sprint State:** Sprint/Executing`. See `skills/sprint-lifecycle.md` for state machine rules.
+
 ### Domain (2 labels)
 
 Cross-plugin integration labels for domain-specific validation gates.

plugins/projman/skills/sprint-lifecycle.md

@@ -0,0 +1,104 @@
+---
+name: sprint-lifecycle
+description: Sprint lifecycle state machine using milestone labels
+---
+
+# Sprint Lifecycle
+
+## Purpose
+
+Defines the valid sprint lifecycle states and transitions, enforced via labels on the sprint milestone. Each projman command checks the current state before executing and updates it on completion.
+
+## When to Use
+
+- **All sprint commands**: Check state on entry, update on exit
+- **Sprint status**: Display current lifecycle state
+
+---
+
+## State Machine
+
+```
+idle -> Sprint/Planning -> Sprint/Executing -> Sprint/Reviewing -> idle
+         (sprint-plan)    (sprint-start)       (review)      (sprint-close)
+```
+
+## State Labels
+
+| Label | Set By | Meaning |
+|-------|--------|---------|
+| *(no Sprint/* label)* | `/sprint-close` or initial state | Idle - no active sprint phase |
+| `Sprint/Planning` | `/sprint-plan` | Planning in progress |
+| `Sprint/Executing` | `/sprint-start` | Execution in progress |
+| `Sprint/Reviewing` | `/review` | Code review in progress |
+
+**Rule:** Only ONE `Sprint/*` label may exist on a milestone at a time. Setting a new one removes the previous one.
+
+---
+
+## State Transition Rules
+
+| Command | Expected State | Sets State | On Wrong State |
+|---------|---------------|------------|----------------|
+| `/sprint-plan` | idle (no Sprint/* label) | `Sprint/Planning` | Warn: "Sprint is in [state]. Run `/sprint-close` first or use `--force` to re-plan." Allow with `--force`. |
+| `/sprint-start` | `Sprint/Planning` | `Sprint/Executing` | Warn: "Expected Sprint/Planning state but found [state]. Run `/sprint-plan` first or use `--force`." Allow with `--force`. |
+| `/review` | `Sprint/Executing` | `Sprint/Reviewing` | Warn: "Expected Sprint/Executing state but found [state]." Allow with `--force`. |
+| `/sprint-close` | `Sprint/Reviewing` | Remove all Sprint/* labels (idle) | Warn: "Expected Sprint/Reviewing state but found [state]. Run `/review` first or use `--force`." Allow with `--force`. |
+| `/sprint-status` | Any | No change (read-only) | Display current state in output. |
+
+---
+
+## Checking State (Protocol)
+
+At command entry, before any other work:
+
+1. Fetch the active milestone using `get_milestone`
+2. Check milestone description for `**Sprint State:**` line
+3. Compare against expected state for this command
+4. If mismatch: display warning with guidance, STOP unless `--force` provided
+5. If match: proceed and update state after command completes its primary work
+
+**Implementation:** Use milestone description metadata. Add/update a line:
+```
+**Sprint State:** Sprint/Executing
+```
+
+This avoids needing actual Gitea labels on milestones (which may not be supported). Parse this line to check state, update it to set state.
+
+---
+
+## Setting State
+
+After command completes successfully:
+
+1. Fetch current milestone description
+2. If `**Sprint State:**` line exists, replace it
+3. If not, append it to the end of the description
+4. Update milestone via `update_milestone`
+
+**Format:** `**Sprint State:** <state>` where state is one of:
+- `Sprint/Planning`
+- `Sprint/Executing`
+- `Sprint/Reviewing`
+- (empty/removed for idle)
+
+---
+
+## Displaying State
+
+In `/sprint-status` output, include:
+
+```
+Sprint Phase: Executing (since 2026-02-01)
+```
+
+Parse the milestone description for the `**Sprint State:**` line and display it prominently.
+
+---
+
+## Edge Cases
+
+- **No active milestone**: State is implicitly `idle`
+- **Multiple milestones**: Use the open/active milestone. If multiple open, use the most recent.
+- **Milestone has no state line**: Treat as `idle`
+- **`--force` used**: Log to milestone: "Warning: Lifecycle state override: [command] forced from [actual] state on [date]"