feat(checkpoints): record agent and intent in the checkpoint name

Zerto's tagged checkpoint insert takes exactly one field. The 10.x
swagger model VpgInsertTagCheckpointDataApi has a single property,
checkpointName, and the 9.0 API reference lists CheckpointName as the
only request value. There is no description field, so who the agent is
and what it is about to do have to live inside the name.

Old name:
  ai:claude:chg-412:20260921T170829Z

New name:
  ai:claude | edit /home/justin/app-config.yaml | vm=jp-ubuntu |
  change=chg-412 | 20260921T170829Z

zerto_create_tagged_checkpoint and zerto_guard_before_mutate take a new
action argument: free text saying what the agent is about to do. The VM
name is filled in from the find result. An operator reading the journal
in the Zerto UI can now see which agent inserted a checkpoint and why,
without the agent transcript.

Field text is sanitised so the name stays one readable line: control
characters and runs of whitespace collapse to single spaces, ';' becomes
',' because Zerto appends "; Used for File Level Restore" to its own
tags, and '|' becomes '/' because ' | ' is our field separator. Capped
at TAG_MAX_LEN (250).

Measured against ZVM 10.x while picking the format:

- names of at least 400 chars are accepted, and spaces, slashes,
  parentheses, '=' and '|' all survive the round trip
- tagged checkpoint inserts fired back to back at one VPG are silently
  dropped. The POST returns 200 and queues a task, but only the first
  checkpoint appears. tag_vpgs already inserts then waits per VPG, so
  it is correct; added a comment so nobody turns that loop into an
  asyncio.gather().

Verified end to end: guard inserted cp 1197 on VPG jp-ubuntu, the name
read back byte-identical from the journal, and FLR from that checkpoint
returned the 158 byte pre-mutation file.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_016yVfC5nvZowoLFnEGWhLGn

tests/test_checkpoints.py

@@ -1,18 +1,46 @@
 from datetime import UTC, datetime
 
-from zerto_rewind_mcp.checkpoints import checkpoint_id, checkpoint_tag, make_tag
+from zerto_rewind_mcp.checkpoints import (
+    TAG_MAX_LEN,
+    checkpoint_id,
+    checkpoint_tag,
+    make_tag,
+)
+
+WHEN = datetime(2026, 9, 21, 15, 4, 5, tzinfo=UTC)
 
 
-def test_tag_format():
-    when = datetime(2026, 9, 21, 15, 4, 5, tzinfo=UTC)
-    tag = make_tag("codex", "chg-99", when)
-    assert tag == "ai:codex:chg-99:20260921T150405Z"
+def test_tag_describes_agent_and_action():
+    tag = make_tag(
+        "claude",
+        "chg-99",
+        WHEN,
+        action="edit /home/justin/app-config.yaml",
+        vm_name="jp-ubuntu",
+    )
+    assert tag == (
+        "ai:claude | edit /home/justin/app-config.yaml | vm=jp-ubuntu "
+        "| change=chg-99 | 20260921T150405Z"
+    )
 
 
-def test_tag_strips_junk():
-    tag = make_tag("agent one", "path/with spaces", datetime(2026, 1, 1, tzinfo=UTC))
-    assert " " not in tag
-    assert tag.startswith("ai:agent-one:path-with-spaces:")
+def test_tag_without_action_or_vm():
+    assert make_tag("codex", "chg-99", WHEN) == "ai:codex | change=chg-99 | 20260921T150405Z"
+
+
+def test_tag_keeps_separators_unambiguous():
+    # ';' is what Zerto appends to its own tags; '|' is our field separator.
+    tag = make_tag("a;b", "c|d", WHEN, action="rm -rf /tmp;x", vm_name="v|m")
+    assert tag.count(" | ") == 4
+    assert ";" not in tag
+
+
+def test_tag_collapses_whitespace_and_caps_length():
+    tag = make_tag("agent one", "chg 1", WHEN, action="do\n  many   things")
+    assert "\n" not in tag
+    assert "do many things" in tag
+    long_tag = make_tag("a" * 200, "b" * 200, WHEN, action="c" * 400)
+    assert len(long_tag) <= TAG_MAX_LEN
 
 
 def test_checkpoint_row_keys():