docs(spec): remove overview.md, rewrite architecture.md for plugin model

## Why
overview.md described the chunk-based delivery model, which is superseded
by the marketplace/plugin pivot. architecture.md was equally stale: it
described .agents/skills/ as the canonical skills source (directory does not
exist), a provider-manifest.sh symlink mechanism (never built), and sync.sh /
init-project.sh as existing scripts (Chunk 6, not yet built).

## Implementation Notes
- overview.md deleted; all cross-references scrubbed from AGENTS.md, CONTEXT.md,
  and three notes/research files
- architecture.md fully rewritten: content deployment model reflects actual
  install.sh behaviour (DEPLOY_FILES / DEPLOY_EXECUTABLES / DEPLOY_DIRS);
  plugin model section added listing all 5 plugins; directory structure section
  removed (was describing a layout that no longer exists)
- "Chunk 6" phase label → "planned"; "chunk workflow" removed from AGENTS.md
  description
- Fixed broken path docs/HUMANS.md → docs/wiki/HUMANS.md in governance layer
  and core/instructions/governance.md

Refs: #15

AGENTS.md

@@ -14,7 +14,6 @@ Read CONTEXT.md at the start of every session in this repo.
 Read these on demand:
 
 - `docs/VISION.md` — purpose, goals, and long-term Management Application vision
-- `docs/spec/overview.md` — current deployed state; what works today
 - `docs/spec/architecture.md` — current directory structure, install pipeline, provider model
 - `docs/adr/` — architectural decisions; read before answering design questions or proposing structural changes
 - `docs/ai-constitution.md` — full governance evidence base; read when a governance decision needs justification