Label color-coding + HANDOFF.md workflow update markdown

[MeatPopSci1972]

Label color-coding + HANDOFF.md workflow update

Part of Issue Contracts (umbrella: #57 ).
Depends on **Issues #58 and #59 ** being complete.

Context

Routing labels (rfc, for_cowork, for_session, regression, hotfix) now carry contracts. They direct AI engagement mode. Visual reinforcement in the GitHub UI helps humans scan the backlog at a glance and reinforces the contract meaning.

This issue also updates HANDOFF.md to document the full AI workflow (verify → knowledge → plan) now that both endpoints exist.

Color mapping

Consistent with common conventions (red = stop/urgent, green = go, yellow = caution, purple = design):

Label	Color	Hex	Meaning
regression	Red	#d73a4a	Drop everything
hotfix	Red	#d73a4a	Drop everything
rfc	Purple	#7057ff	Design spike — think first
for_cowork	Green	#0e8a16	Ready to execute
for_session	Yellow	#fbca04	Discussion required
next-priority	Dark blue	#0052cc	Owner-designated priority
icebox	Gray	#e6e6e6	Parked
architecture	Light purple	#c5b6ff	Cross-cutting design
server	Dark gray	#333333	Ops/infrastructure
ui	Cyan	#1d76db	User-facing rendering
tests	Light green	#b4e59c	Test infra
tour	Orange	#ff8c00	Onboarding feature
accessibility	Pink	#ff69b4	a11y concern
external-assets	Brown	#8b4513	Assets fetched at runtime
store	Navy	#1a3a7c	Store/state layer

HANDOFF.md updates

FIRST ACTIONS section

Add steps 6 and 7:

6. Before planning any issue, run `GET /verify-issue?number=N`. 
   If `ready_to_plan: false`, address errors. If warnings present, 
   acknowledge with user before proceeding.

7. After green verification, run `GET /knowledge?labels=<labels>` 
   (using labels from verify response). Apply principles returned.

DEV SERVER API table

Add rows:

| /verify-issue?number=N | Label-keyed verification + mode derivation |
| /knowledge?labels=X,Y  | Label-keyed principle retrieval            |

Issue Contracts section (new)

Add a new top-level section documenting:

• What an Issue Contract is (the four layers)
• The five routing labels and their modes
• How color coding reinforces contract meaning
• Link to the umbrella issue

Steps

1. Apply colors via gh label edit for each label in the mapping table
2. Update HANDOFF.md FIRST ACTIONS with steps 6 and 7
3. Update HANDOFF.md DEV SERVER API table with new endpoints
4. Add Issue Contracts section to HANDOFF.md
5. Commit HANDOFF.md changes
6. Verify colors render correctly in GitHub UI (screenshot attached to issue)

Done when

• All labels in the color mapping table updated to specified hex values
• HANDOFF.md FIRST ACTIONS includes steps 6 and 7
• HANDOFF.md DEV SERVER API table includes both new endpoints
• HANDOFF.md has a dedicated "Issue Contracts" section
• Screenshot of issue list with new colors attached to this issue for verification

Do not touch

• Any code files (this is documentation + GitHub API only)
• Label names (colors only; names are stable)
• Issue bodies (this issue doesn't relabel or re-triage anything)

Out of scope

• Auto-applying labels based on content
• Bulk re-triage of existing issues
• Renaming labels