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

[s6pa1rta3n-lab]

Summary

Replaces the stale legacy plan reference (docs/SCF_PLAN.md) in docs/SCF_TRANCHE_PLAN.md with a working relative link to the maintained platform plan (docs/PLATFORM_PLAN.md).

Closes #246

Payout Stipulations Checklist

• The tranche plan contains no reference to the removed legacy plan.
• The replacement is a working relative Markdown link (./PLATFORM_PLAN.md).
• Documentation link validation passes (node scripts/check-links.js validates 43 local links, 0 broken).
• No unrelated prose is rewritten.

Changes

• docs/SCF_TRANCHE_PLAN.md: Replaced docs/SCF_PLAN.md reference with the [platform plan](./PLATFORM_PLAN.md) in acceptance criteria item 5.

Verification

• pnpm docs:check-links executed and passed cleanly:
  • 43 local links checked (up from 42 prior to linking)
  • 0 broken links
• pnpm docs:check executed and passed cleanly.
• pnpm logging:check executed and passed cleanly.
• pnpm threat-model:check executed and passed cleanly.

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC