docs(scf): replace the stale legacy plan reference #246

Problem

The SCF tranche plan says detailed background lives in docs/SCF_PLAN.md, but that legacy document was removed. The repository now uses the platform plan for that background.

Task

Update the prose to reference and link to the maintained planning document while preserving the surrounding no-duplication guidance.

Acceptance criteria

• The tranche plan contains no reference to the removed legacy plan.
• The replacement is a working relative Markdown link.
• Documentation link validation passes.
• No unrelated prose is rewritten.

Suggested implementation prompt

Replace the stale legacy-plan reference in the SCF tranche plan with a relative link to the maintained planning document, run the documentation link check, and keep the diff to that prose correction.