Conversations on the new loop (M2.5, PR A)

- A top-level message that isn't a command is a conversation: an item in
  the `general` scope (L1), threaded under the operator's message, with
  `reply`, `ask_human` and `propose_work` and no workspace or execution.
  `general` is reserved as a project name.
- propose_work(project, title, summary) goes up as a revision for a
  go-ahead (or a plain yes). On go, under a file_issue intent (L8), the
  record issue is opened in that project, assigned to the agent and
  carrying the new item's ID; its item starts agreed, with the summary as
  its brief. Intake skips any issue carrying an ID (D1); boot adopts a
  filed issue or puts the proposal up again.
- A conversation closes after tuning.loop.general_idle_s (default a day);
  a reply in its thread then starts a new conversation that continues it.

Tests: conversation unit tests (threading, filing, the marker, an unknown
project, changes, idle close and continuation, a plain yes, both crash
cases, commands at top level); the walk with conversations, proposals and
idle time, and records_have_one_item / conversations_do_no_work; an e2e
conversation that becomes filed work.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPQ14L3LikyefCMFBqEbKE

nisse-dev.example.yaml

@@ -79,3 +79,4 @@ tuning:
     safety_margin: 0.1       # fraction of the window held back
   loop:                      # the new loop (loop: new)
     bug_grace_s: 600         # a `bug` restatement proceeds after this long unanswered
+    general_idle_s: 86400    # a conversation closes after this long without a message