feat(config): store config defaults and provide sparse mapping functions - #6205

Implements CLI-2155: a stored reference of config default values and a mapping function that omits values matching them, so config diff (CLI-2156) and config pull (CLI-2064) can compare sparse configs instead of full effective ones.

Currently in this PR (design docs; implementation follows on this branch):

• ADR 0017 — Sparse Config Subtraction (docs/adr/0017-sparse-config-subtraction.md): pins the design before code lands. Key points:
  • Defaults are derived from the schema's existing default annotations by decoding {} (getDefaultProjectConfig()), not from a hand-written table that would drift.
  • The mapping core is a pure, parameterized subtractProjectConfig(config, baseline) (strict deep equality, order-sensitive arrays, empty sections dropped), with omitDefaultValues(config) as the defaults-baseline entry point.
  • The remote-block baseline rule: subtraction never prunes [remotes.*] against global defaults — a remote value equal to a global default still overrides a differing base value. A remote block's correct baseline is the merged base config (applied by CLI-2156).
  • Scope split: the merge cascade and API-response→config shape translation land with the diff core in CLI-2156, keeping @supabase/config free of Management API types.
• CONTEXT.md: a root glossary establishing the vocabulary the config-driven-development work builds on (default config, sparse config, subtract, remote block, base config, drift).

🤖 Generated with Claude Code