devin plugin: expand ~ in CLAUDE_HOME from env + add tests & ATIF fixture

Review fixes:
- Path bug: SKILLOPT_DEVIN_CLAUDE_HOME (and SKILLOPT_SLEEP_REPO) read from the
  env are now wrapped in os.path.expanduser, so the documented "~/..." config
  no longer passes a literal ~ to --claude-home (which yielded zero mined
  sessions). expanduser on an absolute default is a no-op.
- tests/test_devin_plugin.py: tool-schema completeness, action→subcommand map,
  backend enum, the CLAUDE_HOME expansion regression, and an ATIF-v1.7 harvest
  shape test against a bundled fixture.
- plugins/devin/fixtures/devin_sample.json: sample ATIF-v1.7 transcript.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

plugins/devin/mcp_server.py

@@ -36,14 +36,13 @@ import sys
 
 # ── constants ─────────────────────────────────────────────────────────────────
 
-REPO_ROOT = (
+REPO_ROOT = os.path.expanduser(
     os.environ.get("SKILLOPT_SLEEP_REPO")
     or os.path.abspath(os.path.join(os.path.dirname(__file__), "..", ".."))
 )
 PLUGIN_DIR = os.path.dirname(os.path.abspath(__file__))
-CLAUDE_HOME = os.environ.get(
-    "SKILLOPT_DEVIN_CLAUDE_HOME",
-    os.path.expanduser("~/.skillopt-sleep-devin"),
+CLAUDE_HOME = os.path.expanduser(
+    os.environ.get("SKILLOPT_DEVIN_CLAUDE_HOME", "~/.skillopt-sleep-devin")
 )
 MANAGED_SKILL_NAME = os.environ.get("SKILLOPT_MANAGED_SKILL", "skillopt-sleep-learned")
 PROTOCOL_VERSION = "2024-11-05"