docs: concept annexation, provider/reference docs, ACT-1 groundwork

Mosaic concepts pages now own the adapted content; source/license
metadata under docs/reference/concepts. Adds ACT-1 agent-context
planning capture, pinned concept test package + preparation utility,
foundation observation notes (durability, evidence, federation,
onboarding, workflow), and the #1495 consolidation assessment.
TOOLS.md updated for the host-dev launcher.

docs/concepts/managed-worktrees.md

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+# Managed development worktrees
+
+Status: proposed development lifecycle; no new allocator or cleanup service exists
+as a result of this documentation.
+
+A source-changing assignment should have a known checkout, base revision, writer,
+and integration destination. Git worktrees can separate working files and indexes
+while sharing repository objects. They are not filesystem or credential sandboxes.
+
+## Ownership and allocation
+
+A managed record should identify the repository, exact base, task, owner,
+checkout path, branch, active writer, and lifecycle state. A named checkout is not
+proof of a valid assignment. Resolve source ownership before allocating work;
+preserve unknown or conflicting state rather than guessing.
+
+Check capacity before allocation and setup. Failure must leave clear evidence
+and recoverable state. Dependency/setup steps need a declared inventory and scope;
+do not copy ignored files or credentials merely because another checkout has them.
+
+Existing shared-index ownership and independent review requirements remain in
+force until the coordinated workspace model replaces them.
+
+## Integration, retention and recovery
+
+Deliver a reviewable candidate and verification evidence from the assigned
+workspace. The authorized integrator applies it to the intended destination.
+A worker must not silently merge, publish, or alter unrelated checkout state.
+
+Closing work retires it from ordinary use and preserves evidence. It does not
+authorize deletion. Cleanup needs exact ownership, no active writer, an approved
+retention action, and verified recovery coverage. Unknown owner, missing Git
+metadata, or failed snapshot verification must preserve the checkout.
+
+Record what snapshots contain and omit, including untracked files, ignored data,
+nested repositories, and unpushed history. Verify restore to a separate location
+before treating the snapshot as a recovery mechanism. Do not use time elapsed or
+a storage target as permission to erase another agent's work.
+
+ACT-C03 is a synthetic cleanup recommendation test. Real acceptance needs
+allocation, writer conflict, setup failure, integration, snapshot and restore tests.
+See [ACT-1](../plans/2026-09-07_agent-context-templates-and-migration.md).