[Task] Slim down branching strategy and align with live labeler rules #505

Deliverable

Update branching documentation to explicitly separate required core prefixes from optional profile prefixes and align with current automation.

Scope Boundary

• In scope: documentation clarity and consistency updates.
• Out of scope: changing branch-prefix behaviour in workflows.

Acceptance Checklist

• Required core prefix set is explicit and concise.
• Optional profile prefixes for client/product contexts are explicit.
• References point to canonical spec and .github/labeler.yml.
• No contradictions with current labeler branch regex.

Branch Prefix Expectation

• Use docs/.

Validation Commands

npx markdownlint-cli2 "docs/BRANCHING_STRATEGY.md" "docs/GITHUB_PROJECT_OPERATIONS_SPEC.md"
git diff --check

Touched Paths

• docs/BRANCHING_STRATEGY.md
• docs/GITHUB_PROJECT_OPERATIONS_SPEC.md
• .github/labeler.yml