fix: read codebase conventions during new-project for brownfield projects

[amanape]

What

Read .planning/codebase/CONVENTIONS.md during /gsd:new-project questioning (Step 3) and PROJECT.md creation (Step 4) for brownfield projects.

Why

Brownfield projects have conventions already documented but new-project never reads them — leading to redundant questions (e.g., "where should modules live?" when conventions already specify /lib).

Details

• Step 3: When has_codebase_map is true, read CONVENTIONS.md before opening the conversation
• Step 4: Expand brownfield file reads to include CONVENTIONS.md alongside ARCHITECTURE.md and STACK.md
• Conditional on has_codebase_map — zero cost for greenfield projects

Pros: Fixes the root cause at the earliest decision point. Prevents redundant questions.
Cons: Adds context to orchestrator window during questioning. Only helps if /gsd:map-codebase was run first (expected brownfield flow).

Related: #936 (planner agent), #937 (phase prompt template) — 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