Add behavioral contracts for Squad activation label linkage

[bradygaster]

Parent: #1957

Blocked by: implementation of temporary-ID targeting, fast-path parity, reporting, and capacity safeguards under this epic.

Problem

Existing tests primarily assert prompt phrases and frontmatter. That allowed an invalid assumption about returned issue numbers to pass while CI remained green.

Scope

Add focused tests that verify the compiled gh-aw contract and the linkage invariants for both activation paths.

Success criteria

• Tests inspect compiled safe-output schemas/configuration, not only source prose.
• Tests prove every new create_issue request has a unique temporary ID and its add_labels request targets the same ID.
• Tests fail for omitted targets, implicit triggering-issue targets, duplicate temporary IDs, predicted issue numbers, or dependence on a synchronously returned real number.
• Tests cover existing/reused issue targeting separately.
• Tests cover base labels, roster labels, @copilot, multi-owner epics, non-roster omissions, truthful summaries, reruns, and capacity boundaries.
• Both /squad plan activate and /squad activate are covered.
• Targeted tests and strict compilation pass.

Out of scope

Live GitHub E2E execution and checker distribution.