fix: add codebase conventions to phase prompt template for brownfield executors

[amanape]

What

Add conditional .planning/codebase/CONVENTIONS.md reference to the phase prompt template's <context> section for brownfield projects.

Why

The executor agent that runs PLAN.md files never sees codebase conventions — it may violate existing coding patterns during implementation. This is the last line of defense after questioning (#935) and planning (#936).

Details

• phase-prompt.md template: Add conditional CONVENTIONS.md to <context> section
• phase-prompt.md docs: Update "Parallel-aware context" example to match

Follows the template's existing pattern for conditional context (commented with # If .planning/codebase/ exists).

Pros: Last backstop — even if planning misses conventions, executor sees them. Minimal change (3 lines each in template + docs).
Cons: Lowest impact of the three PRs — if planner already respects conventions, executor inherits through well-written tasks. Template comments are guidance for the planner, not enforced.

Related: #935 (new-project questioning), #936 (planner agent) — together these ensure CONVENTIONS.md flows through questioning → planning → execution.

Testing

• Tested on macOS
• Tested on Windows
• Tested on Linux

Checklist

• Follows GSD style (no enterprise patterns, no filler)
• Updates CHANGELOG.md for user-facing changes
• No unnecessary dependencies added
• Works on Windows (backslash paths tested)

Breaking Changes

None