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overview.md described the chunk-based delivery model, which is superseded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

docs/notes/factory-research-gaps-conflicts.md

@@ -49,7 +49,7 @@ Factory treats `LESSONS.md` as a required committed file — the mechanism for l
 
 ### 3.2 `docs/spec/` living spec layer
 
-Factory requires `docs/spec/overview.md` and `docs/spec/architecture.md` — a spec that is updated in the same PR as any behaviour change. The current docs structure has no equivalent layer; workflow artifacts go in `docs/prd/`, `docs/ard/`, etc. Adding `docs/spec/` is additive (not conflicting), but it changes the docs convention and needs to be decided before Chunk 4 (workflows) defines workflow artifacts.
+Factory requires `docs/spec/architecture.md` — a spec that is updated in the same PR as any behaviour change. The current docs structure has no equivalent layer; workflow artifacts go in `docs/prd/`, `docs/ard/`, etc. Adding `docs/spec/` is additive (not conflicting), but it changes the docs convention and needs to be decided before Chunk 4 (workflows) defines workflow artifacts.
 
 ### 3.3 Session handoff skill (impacts Chunk 3)