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/agents.go

@@ -0,0 +1,130 @@
+package seed
+
+// AgentsMD returns the canonical AGENTS.md content for a new workspace.
+// This is the single source of truth for agent-facing instructions in
+// v0.6+. Agent-specific files (CLAUDE.md, etc.) become thin shims
+// pointing back here. Per v0.6 spec §6.
+//
+// isProject controls a small set of project-specific additions
+// (workspace-structure section, git conventions).
+func AgentsMD(isProject bool) string {
+	base := `# Workspace Instructions
+
+This workspace uses ctask. AGENTS.md is the canonical instruction file
+for agents working here. Agent-specific files (CLAUDE.md, etc.) are
+thin shims that defer to this document.
+
+## Session Workflow
+
+- Read handoff.md first when starting a session. It carries the
+  current-state briefing: last completed work, immediate next step,
+  blockers, and files to inspect first.
+- Update notes.md with decisions, design rationale, and observations
+  during the session.
+- Update handoff.md before ending the session so the next agent (or
+  the next instance of you) can pick up cleanly. Keep it short and
+  current -- handoff.md is not a history file.
+
+## Notes Archival
+
+When notes.md becomes too large to scan comfortably (roughly 300-500
+lines or 10-20 KB), archive older completed-phase sections:
+
+1. Create a new file: context/notes-archive/YYYY-MM-DD-topic.md
+2. Move the older sections into it, preserving them exactly.
+3. Add a pointer at the top of notes.md:
+   "Older notes archived in context/notes-archive/YYYY-MM-DD-topic.md"
+4. Do not delete or summarize-away historical notes as the only
+   preservation mechanism.
+5. Keep handoff.md short and current. It is not a history file.
+
+## Cross-Workspace Context
+
+Related work may exist in other ctask workspaces. Use:
+
+  ctask list --all          discover all workspaces, including archived
+  ctask info <workspace>    view metadata and status
+  ctask notes <workspace>   read another workspace's notes.md
+  ctask path <workspace>    get the filesystem path
+
+Treat other workspaces as read-only unless the user explicitly asks
+otherwise.
+
+## Do Not Touch
+
+- Do not modify .ctask/ -- those are ctask internals (lease, manifests,
+  session summary, write lock).
+- Do not edit task.yaml's metadata fields by hand. ctask owns them.
+`
+	if !isProject {
+		return base + `
+## File Placement (task workspace)
+
+- 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 in the workspace root.
+`
+	}
+	return base + `
+## Workspace Structure (project workspace)
+
+This workspace uses ctask's project layout. The workspace root contains
+ctask management files; your project code lives in the project
+subdirectory (named after the workspace slug). When working on project
+code, operate inside the project subdirectory.
+
+- Workspace root: ctask metadata, session logs, notes, reference material
+- Project subdirectory: source code, project-specific configuration
+- context/: reference material and imported specs (workspace level)
+- output/: deliverables and exports (workspace level)
+- logs/: session logs (managed by ctask)
+
+## Git Conventions
+
+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 elsewhere -- the repo root is the workspace root.
+`
+}
+
+// ClaudeShimMD returns the thin CLAUDE.md generated for new workspaces
+// whose agent.type is "claude". It exists purely to point Claude Code at
+// AGENTS.md; canonical instructions live there. Per v0.6 spec §6.
+func ClaudeShimMD() string {
+	return `<!-- Generated by ctask. Edit AGENTS.md for canonical instructions.
+     This file may be regenerated by future ctask agent-management commands. -->
+
+# Claude Code Instructions
+
+This workspace uses AGENTS.md as the canonical agent guidance file.
+Read AGENTS.md first.
+
+## Claude-specific notes
+
+- Use claude CLI conventions for file operations.
+- Respect the workspace structure described in AGENTS.md.
+`
+}
+
+// HandoffMD returns the minimal handoff.md template seeded into new
+// workspaces. The agent fills it in at session end; the next session
+// reads it first. Per v0.6 spec §8.
+func HandoffMD() string {
+	return `# Handoff
+
+## Current state
+
+New workspace. No work completed yet.
+
+## Next step
+
+[Describe the task or project goal here.]
+
+## Files to read first
+
+- AGENTS.md - workspace instructions and conventions
+`
+}