docs: update planning PRDs for BRD and PRD builder output path change

[github-actions]

Component: Documentation

Bug Description:
PR #2098 moved BRD and PRD output files from docs/brds/ and docs/prds/ to docs/project-planning/. The internal planning PRD documents for the BRD Builder and PRD Builder agents still reference the old output directories.

Files affected and stale references:

File	Stale Reference	Should Be
docs/planning/prds/brd-builder-agent-prd.md	Line 129: docs/brds/<kebab-case-name>-brd.md	docs/project-planning/<kebab-case-name>-brd.md
docs/planning/prds/brd-builder-agent-prd.md	Line 222: acceptance criterion AC-001 references docs/brds/	docs/project-planning/
docs/planning/prds/prd-builder-agent-prd.md	Line 109: user story references docs/prds/	docs/project-planning/

Expected Behavior:
The BRD Builder Agent PRD's functional requirement for "BRD Persistence" (line 129) and acceptance criterion AC-001 (line 222) should reference docs/project-planning/ as the BRD output location. The PRD Builder Agent PRD's user story for "PRD Creation" should reference docs/project-planning/ as the PRD output location.

Steps to Reproduce:
Introduced in commit b5c695f — PR #2098. The brd-builder.agent.md diff changed File locations from docs/brds/<kebab-case-name>-brd.md to docs/project-planning/<kebab-case-name>-brd.md. The prd-builder.agent.md diff changed PRD creation from docs/prds/<kebab-case-name>.md to docs/project-planning/<kebab-case-name>.md.

Additional Context:

• Changed agents: .github/agents/project-planning/brd-builder.agent.md, .github/agents/project-planning/prd-builder.agent.md
• These are internal planning documents; the agent-facing doc docs/agents/project-planning/brd-prd-builders.md only references session state directories (.copilot-tracking/brd-sessions/, .copilot-tracking/prd-sessions/) which are unchanged and accurate.

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