feat(v0.6): AGENTS.md seed + CLAUDE.md shim + handoff + context-archive scaffold

New workspaces get:
  - AGENTS.md (always): canonical agent instructions — handoff workflow,
    notes-archive convention (~300-500 line trigger), cross-workspace
    discovery, do-not-touch warnings. Project variant adds workspace-
    structure and git-conventions sections.
  - CLAUDE.md shim (claude type only per v0.6 spec §6 — opencode shim
    deferred until its instruction-file convention is verified).
  - handoff.md: minimal current-state template the agent updates per
    session.
  - context/notes-archive/.gitkeep: directory pinned for git tracking,
    ready for the agent to populate per the archive convention.

seed.ClaudeMD and seed.ClaudeMDProject removed — no callers remain.
Existing workspaces are NOT modified; this is strictly a ctask-new code
path. The seed-wins overlay rule still applies — a user seed dir's
AGENTS.md/CLAUDE.md overrides the built-in.

internal/seed/templates.go

@@ -2,131 +2,6 @@ package seed
 
 import "fmt"
 
-// ClaudeMD returns the built-in default CLAUDE.md content for a task workspace.
-// Parameters are accepted for API compatibility but are not interpolated in v0.3.
-func ClaudeMD(slug, category, workspacePath string) string {
-	_ = slug
-	_ = category
-	_ = workspacePath
-	return `# Workspace Guidelines
-
-This is a ctask workspace. Prefer operating inside this directory unless explicitly instructed otherwise.
-
-## File Placement
-
-- Source code and scripts -> workspace root or ` + "`src/`" + `
-- Documentation, summaries, reports -> ` + "`docs/`" + `
-- Deliverables and exports -> ` + "`output/`" + `
-- Reference material and imported files -> ` + "`context/`" + `
-- Do not place non-code outputs (docs, summaries, exports) in the workspace root
-
-## Conventions
-
-- Do not install global packages or modify system files unless asked
-- Record important assumptions and actions in notes.md
-- Keep the workspace root clean -- use subdirectories for organization
-
-## Cross-Workspace Context
-
-Related work may exist in other ctask workspaces. For project continuation,
-migration, debugging, or building on prior work, inspect related workspaces
-before making changes.
-
-Available commands:
-
-  ctask list --all          discover all workspaces, including archived
-  ctask info <workspace>    view metadata and status of any workspace
-  ctask notes <workspace>   read another workspace's notes.md
-  ctask path <workspace>    get the filesystem path to inspect files directly
-
-Treat other workspaces as read-only unless the user explicitly asks you to
-modify them.
-
-## Session Handoff
-
-Before ending a session, append a brief summary to notes.md with:
-
-- What was accomplished
-- Key decisions made
-- Open follow-ups or unfinished work
-- How to continue from here
-
-Keep it concise -- a few bullet points is enough.
-`
-}
-
-// ClaudeMDProject returns the built-in default CLAUDE.md content for a project workspace.
-func ClaudeMDProject() string {
-	return `# Project Workspace Guidelines
-
-This is a ctask project workspace -- a long-lived working environment, not a disposable task.
-
-## Workspace Structure
-
-This is a ctask project workspace. The workspace root contains ctask management
-files. Your project code lives in the project subdirectory.
-
-- Workspace root: ctask metadata, session logs, notes, reference material
-- Project subdirectory: source code, project CLAUDE.md, project configuration
-- ` + "`context/`" + `: reference material and imported specs (workspace level)
-- ` + "`output/`" + `: deliverables and exports (workspace level)
-- ` + "`logs/`" + `: session logs (managed by ctask)
-
-When working on project code, operate inside the project subdirectory.
-Place project-specific CLAUDE.md, documentation, and configuration there.
-
-## File Placement
-
-- Source code -> ` + "`src/`" + ` (inside the project subdirectory)
-- Documentation -> ` + "`docs/`" + ` (workspace level for general notes, project subdir for project docs)
-- Deliverables and exports -> ` + "`output/`" + ` (workspace level)
-- Reference material -> ` + "`context/`" + ` (workspace level)
-- Tests -> ` + "`tests/`" + ` (inside the project subdirectory)
-- Configuration files -> inside the project subdirectory
-- Do not place non-code outputs in the workspace root
-
-## Conventions
-
-- This project uses git. Commit meaningful changes with clear messages.
-- Do not install global packages or modify system files unless asked.
-- Record important assumptions and actions in notes.md (workspace level).
-- Keep the workspace root clean.
-
-## Git
-
-This workspace uses a single git repository initialized at the workspace root.
-The project subdirectory and all its contents are tracked by this root repo.
-Do not initialize additional git repositories inside the project subdirectory
-or any other subdirectory. If you need to check git status or make commits,
-the repo root is the workspace root.
-
-## Cross-Workspace Context
-
-Related work may exist in other ctask workspaces. For project continuation,
-migration, debugging, or building on prior work, inspect related workspaces
-before making changes.
-
-Available commands:
-
-  ctask list --all          discover all workspaces, including archived
-  ctask info <workspace>    view metadata and status of any workspace
-  ctask notes <workspace>   read another workspace's notes.md
-  ctask path <workspace>    get the filesystem path to inspect files directly
-
-Treat other workspaces as read-only unless the user explicitly asks you to
-modify them.
-
-## Session Handoff
-
-Before ending a session, append a brief summary to notes.md with:
-
-- What was accomplished
-- Key decisions made
-- Open follow-ups or unfinished work
-- How to continue from here
-`
-}
-
 // NotesMD returns the skeleton notes.md content.
 func NotesMD(title string) string {
 	return fmt.Sprintf(`# %s