fix(agent): bound spreadsheet and media generation

[iamjr15]

Why

Routine spreadsheet requests were falling into a multi-step Python/LibreOffice path because the native XLSX generator could not represent formulas. Image requests also shared one strict schema with video-only fields, causing model adapters to materialize incompatible placeholders and enter validation retry loops.

What changed

• add typed workbook-local formula cells with cached values and standard number formats to the bounded XLSX generator
• route routine workbooks through one native publishing call while retaining the custom Office path for complex work
• replace the mixed media tool with separate strict image and video capabilities
• align capability policy, activity copy, system guidance, skill bundles, and architecture documentation
• remove all legacy mixed-media tool references; no compatibility shim

Architecture and migration effects

• tool capability contract changes from one mixed media capability to two mode-specific capabilities
• no database migration or environment change
• Google AI keys remain request-scoped and lazily resolved

Verification

• pnpm lint
• pnpm typecheck
• pnpm turbo build --force
• pnpm deadcode (existing Knip configuration hint only)
• pnpm architecture:check
• pnpm turbo skills:build
• parsed a representative bakery workbook with total and percentage formula cells
• verified an external-network spreadsheet formula is rejected
• production acceptance will repeat the same ordinary-user spreadsheet and image prompts after the exact merge commit is deployed