feat: promote priority, estimate, assignees to first-class fields

[ulascanzorer]

Summary

Task Reference: [MYMR-190]

Promotes priority, estimate, and assignees from scattered / tag-encoded state to first-class task columns. Schema adds nullable tasks.priority (text), tasks.estimate (integer), and a task_assignees(task_id, user_id) junction. The data migration mirrors existing priority strings from tasks.tags into the new column but intentionally leaves them in tags for branch compatibility during the dual-life period; the strip is deferred to MYMR-195.

The four-dimension tag taxonomy becomes three (work-type, cross-cutting, tech) plus the new priority field; PRIORITY_TAGS and the priority branch in tagTaxonomyHints are removed.

Follow-up: workspace UI changes tracked in MYMR-194; legacy-tag strip in MYMR-195.

Type of change

• New feature
• Bug fix
• Refactor / cleanup
• Documentation

Testing

• Linting passes (bun run lint)
• Typecheck passes (bun run typecheck)
• bun test: 103 / 103 pass, including new coverage for non-member rejection, append vs replace assignee modes, and priority + estimate persistence
• Tested locally with bun run dev (deferred — local DB is on an old schema; verified against Neon via scripts/apply-mymr-190.ts instead)

Notes for reviewer

• Schema enforcement matches MYMR-98 precedent: text().$type<>() / integer().$type<>() narrowing, no pgEnum or DB CHECK. Validation lives at the Zod edge.
• Priority value vocabulary kept 1:1 with the existing tag values (release-blocker, core, normal, backlog) instead of the low/normal/high/release-blocker set originally drafted in AC 1, by user decision during planning.
• Team-scope check on assignee writes lives in assertAssigneesInTeam inside the same transaction as the junction insert, so a concurrent membership revoke cannot slip past.
• Composite PK on (task_id, user_id) in task_assignees (no ordering column). Sets the convention for MYMR-136's twin junction work on acceptanceCriteria + decisions.
• Migration applied to Neon via scripts/apply-mymr-190.ts (idempotent): 472 of 493 tasks now carry a priority value; 21 had no priority tag and stay NULL. The script is left in place for re-runs.
• AC 2 ("removes them from the tags array; no task ends up with both") is intentionally unchecked. The strip is deferred to MYMR-195 once every active branch has merged the new surface; until then, code on main keeps reading priority from tasks.tags via the still-active PRIORITY_TAGS enforcement on that branch.