Add feasibility reports for IDEAS.md

[Copilot]

IDEAS.md lists multiple Flat Email execution modes but lacked per-idea feasibility analysis. This adds a dedicated feasibility report set to make tradeoffs, dependencies, risks, and sequencing easier to review.

• Report index

  • Added feasibility/README.md as a navigable index grouped by idea category.

• Delivery and storage modes

  • Added reports for filesystem, Git-backed archive, cloud storage, encrypted vault, and database/index sink modes.

• Application and integration modes

  • Added reports for desktop/mobile packaging, MCP, local HTTP API, daemon scheduling, container deployment, and SDK exposure.

• Connector modes

  • Added reports for additional providers, local mail-store imports, and push/streaming ingest.

[Copilot]

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