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overview.md described the chunk-based delivery model, which is superseded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

CONTEXT.md

@@ -14,7 +14,7 @@ description: Domain language and decisions for the global AI development config
 `core/instructions/<topic>.md` files are plain markdown — no frontmatter, no schema. The agent decides when to read each file based on task context and the content index label in `providers/claude-code/CLAUDE.md`. Frontmatter is deferred until there is evidence that agents are loading the wrong files in practice.
 
 ### Repo/gitea as source of truth
-All project state, decisions, context, and working conventions live in this repo or Gitea. External memory systems should not be used for this project — they create a split-brain risk where cached state diverges from the repo. At the start of every session, read `CLAUDE.md`, `CONTEXT.md`, `docs/VISION.md`, and `docs/spec/overview.md`. Everything needed to orient is here.
+All project state, decisions, context, and working conventions live in this repo or Gitea. External memory systems should not be used for this project — they create a split-brain risk where cached state diverges from the repo. At the start of every session, read `CLAUDE.md`, `CONTEXT.md`, and `docs/VISION.md`. Everything needed to orient is here.
 
 Before answering any design or architecture question, check for existing decisions: `docs/adr/` (hard architectural decisions).