Add detailed SPEC.md feature specification

[mosesman831]

What this does

Adds SPEC.md — a detailed, technically-grounded feature specification that is the how counterpart to ROADMAP.md's what/when. Docs-only; no code changes.

Grounded in the actual codebase (pipeline, SQLite index schema, MCP server, dashboard, queue, corrections), so it extends rather than re-proposes what already ships.

Contents

• Guiding principles (vault is source of truth, plain files, write/read separation, provider-agnostic, safe-by-default, local-first, cost-bounded).
• 7 epics with per-feature specs (problem, design incl. schema/config/CLI/MCP/HTTP surface, acceptance criteria, risk/effort):
  • A — Capture everywhere: email, browser extension/bookmarklet, Slack/Discord, Obsidian plugin, importers.
  • B — Agent interface: remote MCP (HTTP/SSE), gated capture_note, vault-wide RAG ask, saved searches/timeline.
  • C — Knowledge quality: per-compartment templates, note growth cap + auto-summary/split, entity registry & knowledge graph, merge-back, flat mode, first-class tasks, resurfacing/spaced review, multilingual/PII.
  • D — Quality & ops: golden eval set + CI accuracy gate, observability & cost governance, doctor upgrades, backup/versioning/encryption, reliability hardening.
  • E — Multi-user & privacy.
  • F — Distribution & DX (npm/Docker/Homebrew, init upgrades, docs).
  • G — Extensibility (plugin/hook architecture).
• Consolidated surface changes, cross-cutting config additions (all optional, backward-compatible), testing strategy, phased delivery mapped to roadmap versions, and open questions.
• Cross-linked from ROADMAP.md.

Notes

• Every proposed config key is optional with safe defaults; existing dendrite.config.yaml keeps working.
• Design intent: additive, backward-compatible, behind config flags, with an eval/test suite per epic.

This is a planning artifact intended to be discussed and reprioritized — see the Open questions section.