[frectrium]

What

Adds docs/design/decision-log.md: the template, how to add an entry, and the first 15 entries (DDL-001 to DDL-015) seeded from the exercise plan (#14). Also links the "Design decisions are recorded" box in the PR template to the log.

Closes #17

Why

The brief asks for a Design Decision Log by the end of Week 2. Every Satchel choice needs its Magic Pocket counterpart, Dropbox's reasoning and a source, and #27 turns this file into a report chapter in Week 4.

How to check it

• Each entry has all seven fields from [Docs] Create the Design Decision Log doc #17, and every quote or claim about Dropbox has a Source Log ID. Our own reasoning is marked (our inference).
• DDL-015 (EC2 instance types) is the only deliberate deviation. Its prices aren't sourced yet. [Infra] AWS account and Terraform groundwork #33 adds them.
• All entries are Proposed. They become Accepted when [Design] Finalize the exercise plan, including the erasure coding scheme and parameters #14 merges.

Merge order

This PR, the exercise plan (#56) and the architecture doc (#57) link to each other. Merge this one first, then #56, then #57. Links between them are broken until all three are in.

Checklist

• cargo fmt, cargo clippy and cargo test pass locally (code PRs)
• Tests added or updated for the change (code PRs)
• New sources are added to docs/sources/README.md (research PRs)
• Design decisions are recorded in the Design Decision Log
• Docs and README are updated if behaviour or setup changed