docs(governance): add merge-method decision matrix to branching strategy

[T72]

Pull Request

Linked Issue

Closes #78

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Type

• bug
• refactor
• feature
• architecture
• governance
• infra
• performance
• security
• test
• documentation
• verification

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Summary

Adds a canonical merge-method decision matrix and rationale to the branching strategy so contributors can consistently choose the correct merge strategy by branch pair.

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TDD Checklist (Required)

• New behavior has tests (unit or contract).
• Bug fixes include a regression test.
• Refactors preserve behavior (tests protect it).
• No skipped tests introduced.
• CI is green.

If no tests were added, explain why:
Documentation-only governance clarification; no runtime behavior changes.

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Architectural Integrity Checklist

• No core-plugin coupling introduced.
• No unintended public API contract changes.
• Provider abstraction remains isolated.
• Middleware ordering preserved (if modified).
• Breaking change? (If yes, ADR required.)

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Issue Hygiene Checklist

• Linked issue exists and matches this PR scope.
• If linked issue contains blocked-by, status labels are correct.

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Acceptance Criteria Verification

• Criterion 1: Explicit method by branch pair is documented. Added in docs/developer/branching-strategy.md under "Merge Method Decision Matrix (Canonical)".
• Criterion 2: Why each method is used is documented. Added rationale bullets directly under the matrix.
• Criterion 3: Guidance aligns with existing governance. Content aligns with current branch model and linear main-to-develop sync runbook.

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Risk Notes (Optional)

• Performance impact: none.
• Cost impact: none.
• Latency impact: none.
• Migration required: none.

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Additional Notes

This update is intentionally placed in the developer-facing branching strategy doc to keep merge-policy guidance centralized.