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overview.md described the chunk-based delivery model, which is superseded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

core/instructions/governance.md

@@ -70,7 +70,7 @@ When asked to perform a well-defined, repeatable task — file processing, deplo
 
 ## What This File Does Not Govern
 
-Human process decisions are outside agent scope: oversight checkpoints, human approval gates, post-mortems, regulatory notifications, IP licence scanning, and sustainability measurement. These are defined in `docs/ai-constitution.md` and executed by humans following `docs/HUMANS.md`.
+Human process decisions are outside agent scope: oversight checkpoints, human approval gates, post-mortems, regulatory notifications, IP licence scanning, and sustainability measurement. These are defined in `docs/ai-constitution.md` and executed by humans following `docs/wiki/HUMANS.md`.
 
 The deterministic enforcement layer — pre-commit hooks, CI gates, scanner configuration, audit logging infrastructure, and AI agent permission scoping — is specified in `docs/research/governance_principles/CONTROLS.md` and implemented by humans. Agent instructions alone cannot enforce what deterministic tooling must enforce.