fix(tasks): fail loud when the abstract seam is mounted directly

Review finding (Codex round 1): abstract erases at runtime and
@deepseek-ai/dsh-tasks used to be the mountable registry, so a stale
composition row would register a ctx.tasks with no method implementations
and fail far from the misconfiguration. The seam constructor now rejects
direct mounts with a load-time pointer at dsh-tasks-local; the seam suite
pins the fence, the Agent Note cost paragraph records the actual behavior,
and the stale tool-pty README requirement line names the implementation
package.

docs/core-data-structures/bash.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write
-bash.md: 35cf2061588907dde41123efb01e453eb9cc929d
-bash.zh.md: 0cfeb9e1a858f7057e720215c41a757588751122
+bash.md: e092bfb1bf1aa23f7eeeda41a16a1625d1698201
+bash.zh.md: 7ae60b9f38e6ca5c72e349e141132ae6ec72913d