Remove planning artifacts, scratch files, and spec residue

[nficano]

Strip everything left over from the build process that doesn't belong in a released SDK.

Scope

• Delete planning docs, design notes, refactor checklists, TODO scratchpads
• Remove references to "phase 1/2/3", "step N", "v2 spec", "old spec", etc. in code, comments, and docs
• Strip -- block comments (scratch comments left during development)
• Remove dead branches and feature flags tied to obsolete iterations
• Audit .gitignore for paths that no longer need to be ignored

Blocked by

#3 (do after docs so we don't accidentally nuke something docs reference)

Labels

cleanup, tech-debt