feat(flush): add /flush skill and SessionStart(clear) resume hook

Flush is the checkpoint half of the wrap/restart rhythm. It refreshes the session-context anchor in place, the user runs /clear, and the session resumes from the anchor instead of starting cold. One logical session stays alive across a /clear boundary without the archive-and-commit of wrap-it-up or the full cold boot of startup, which buys cheaper tokens and a sharper context window.

The mechanism splits into two halves around /clear, which wipes the conversation so nothing runs straight through it. The /flush skill is the pre-clear half: dump live state, refresh the anchor's Summary, append a dated flush marker, verify the write landed, then prompt the user to /clear. The agent can initiate at a clean task boundary on its own judgment, but /clear is user-only, so the agent does the work and the user supplies the single keystroke. The session-clear-resume.sh hook is the post-clear half, a SessionStart matcher=clear hook that points the fresh session at the anchor to resume, or at startup when no anchor exists.

I packaged the pre-clear half as a skill rather than a project-workflow doc so both halves are global. The hook was already global, so /flush is now callable by name from any project with no per-project sync.

The hook is canonicalized under hooks/ and symlinked into ~/.claude/hooks/, matching precompact-priorities.sh. settings.json wires the SessionStart entry, and settings-snippet.json carries it so a fresh machine wires the hook on make install-hooks.

2 files changed, 63 insertions, 0 deletions

diff --git a/hooks/session-clear-resume.sh b/hooks/session-clear-resume.sh
new file mode 100755
index 0000000..6692f54
--- /dev/null
+++ b/hooks/session-clear-resume.sh
@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# SessionStart(clear) hook: after /clear, point the fresh context at the
+# session-context anchor so a "flush" resumes the same logical session
+# instead of starting cold.
+#
+# Pairs with the /flush skill (rulesets flush/SKILL.md): the skill's
+# pre-clear half refreshes .ai/session-context.org in place; this hook is the
+# post-clear half that /clear's blank session can't run itself.
+#
+# General by design — not flush-specific:
+#   - anchor present  -> resume (read it, say "flushed.", continue)
+#   - anchor absent   -> run the startup workflow (covers a stray /clear)
+#
+# SessionStart hooks reach the model ONLY through
+# hookSpecificOutput.additionalContext — plain stdout goes to the debug log.
+# After /clear the injected context loads but the model still waits for the
+# user's next message before it acts; that one keystroke is expected.
+#
+# Wire in ~/.claude/settings.json:
+#
+#   {
+#     "hooks": {
+#       "SessionStart": [
+#         {
+#           "matcher": "clear",
+#           "hooks": [
+#             { "type": "command",
+#               "command": "~/.claude/hooks/session-clear-resume.sh" }
+#           ]
+#         }
+#       ]
+#     }
+#   }
+set -euo pipefail
+
+# Resolve the AI_AGENT_ID-aware anchor path — mirror of
+# .ai/scripts/session-context-path, evaluated against the project cwd the
+# hook inherits. Fall back to the singleton path when the id is unset.
+id="${AI_AGENT_ID:-}"
+if [ -n "$id" ]; then
+    safe=$(printf '%s' "$id" | tr -c 'A-Za-z0-9._-' '_')
+    sc=".ai/session-context.d/${safe}.org"
+else
+    sc=".ai/session-context.org"
+fi
+
+if [ -f "$sc" ]; then
+    ctx="A context flush just happened (/clear). The session-context anchor is at ${sc}. Read it now — the * Summary (Active Goal, Next Steps) plus the most recent * Session Log entries — then reply with 'flushed.' on its own line, followed by one line restating the Active Goal and the immediate Next Step, and resume that work. This is a mid-session resume, NOT a fresh start: do not run the startup workflow, do not re-pull rulesets or re-sync templates."
+else
+    ctx="Session started with no session-context anchor present (.ai/session-context.org absent). Run the startup workflow at .ai/workflows/startup.org."
+fi
+
+# Build the whole object in python3 so the additionalContext string is
+# JSON-escaped correctly regardless of its content.
+python3 -c 'import json,sys; print(json.dumps({"hookSpecificOutput":{"hookEventName":"SessionStart","additionalContext":sys.argv[1]}}))' "$ctx"
diff --git a/hooks/settings-snippet.json b/hooks/settings-snippet.json
index 11459f2..1be5f00 100644
--- a/hooks/settings-snippet.json
+++ b/hooks/settings-snippet.json
@@ -1,5 +1,13 @@
 {
   "hooks": {
+    "SessionStart": [
+      {
+        "matcher": "clear",
+        "hooks": [
+          { "type": "command", "command": "~/.claude/hooks/session-clear-resume.sh" }
+        ]
+      }
+    ],
     "PreCompact": [
       {
         "hooks": [