feat(team-mode): add core types (discriminatedUnion for members, D-41/D-42)

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.sisyphus/evidence/team-mode/task-2-category-prompt-required.txt

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+bun test v1.3.12 (700fc117)
+
+ 1 pass
+ 3 filtered out
+ 0 fail
+ 1 expect() calls
+Ran 1 test across 1 file. [64.00ms]

.sisyphus/evidence/team-mode/task-2-disc-both.txt

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+bun test v1.3.12 (700fc117)
+
+ 1 pass
+ 3 filtered out
+ 0 fail
+ 1 expect() calls
+Ran 1 test across 1 file. [65.00ms]

.sisyphus/evidence/team-mode/task-2-disc-category.txt

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+bun test v1.3.12 (700fc117)
+
+ 1 pass
+ 3 filtered out
+ 0 fail
+ 3 expect() calls
+Ran 1 test across 1 file. [69.00ms]

.sisyphus/evidence/team-mode/task-2-eligibility-registry.txt

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+bun test v1.3.12 (700fc117)
+
+ 1 pass
+ 3 filtered out
+ 0 fail
+ 12 expect() calls
+Ran 1 test across 1 file. [61.00ms]

.sisyphus/notepads/team-mode/learnings.md

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+## 2026-04-18 Task 2: types module
+
+- `MemberSchema` needs `.strict()` on the base shape so the discriminatedUnion rejects members that mix `category` and `subagent_type`.
+- `backendType` and `isActive` defaults are part of the schema contract, so tests should use `toMatchObject` instead of exact object equality.
+- The eligibility registry must preserve the plan strings verbatim, especially the hard-reject messages for Momus verification.