feat(session-handoff): add PostToolUse commit-detector hook [v0.3.0]

Hooks live in skills/session-handoff/hooks/:
- commit-detector.ps1  Windows/PowerShell
- commit-detector.sh   POSIX (python3 for JSON parsing)
- README.md            opt-in instructions, cross-platform settings.json
                       snippets, smoke procedure

Hook reads PostToolUse stdin, detects substantive `git commit` (prefix
not in meta/docs/style/chore + fix typo, AND body >200 chars OR files >3).
On hit, emits hookSpecificOutput.additionalContext so Claude Code
surfaces a system reminder next iteration. Silent skip on --amend, failed
commits, non-git cwd, trivial prefix, below thresholds.

install.sh deliberately does NOT mutate settings.json — opt-in via the
README hook config snippet, applied once per machine. SKILL.md body
mentions the hook as an optional alternative to the agent-side
behavioral heuristic.

Bump 0.2.1 -> 0.3.0 MINOR (new opt-in capability, backward-compatible —
SKILL keeps working without the hook).

Deferred (separate follow-ups if needed):
- live-hook e2e smoke (would interfere with current session commits)
- rebase/cherry-pick batch deduplication

Closes [session-handoff-posttooluse-hook] (partial: stdin smoke,
live-hook deferred).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

skills/session-handoff/hooks/commit-detector.ps1

@@ -0,0 +1,69 @@
+#!/usr/bin/env pwsh
+# session-handoff PostToolUse hook (PowerShell).
+#
+# Reads PostToolUse JSON from stdin, detects whether the just-completed
+# Bash tool call was a substantive `git commit`, and on hit emits JSON to
+# stdout with `additionalContext` so Claude Code surfaces a system reminder
+# in the next agent iteration ("substantive commit — consider session-handoff
+# write-mode").
+#
+# Substantive heuristic (mirrors session-handoff SKILL.md):
+#   prefix NOT in (meta:|docs:|style:|chore:|fix typo) AND
+#   (body > 200 chars OR files > 3)
+#
+# Silent skip on: malformed JSON, no command, --amend, failed commit,
+# non-git cwd, trivial prefix, below thresholds. Never blocks the tool call
+# (PostToolUse cannot, by design).
+
+$ErrorActionPreference = 'Stop'
+
+# Read stdin
+try {
+    $raw = [Console]::In.ReadToEnd()
+    if ([string]::IsNullOrWhiteSpace($raw)) { exit 0 }
+    $hook = $raw | ConvertFrom-Json -ErrorAction Stop
+} catch {
+    exit 0
+}
+
+# Only Bash tool, only git commit (not --amend)
+$cmd = $hook.tool_input.command
+if (-not $cmd) { exit 0 }
+if ($cmd -notmatch '(?<![\w-])git\s+commit(?![\w-])') { exit 0 }
+if ($cmd -match '(?<![\w-])git\s+commit\b.*--amend') { exit 0 }
+
+# Only on successful commit
+if ($null -ne $hook.tool_response.exit_code -and $hook.tool_response.exit_code -ne 0) { exit 0 }
+
+# Resolve cwd; require a git work-tree
+$cwd = $hook.cwd
+if (-not $cwd) { $cwd = (Get-Location).Path }
+$inside = & git -C $cwd rev-parse --is-inside-work-tree 2>$null
+if ($inside -ne 'true') { exit 0 }
+
+# Parse last commit
+$subject = (& git -C $cwd log -1 --format='%s').Trim()
+$body    = (& git -C $cwd log -1 --format='%b')
+$files   = ((& git -C $cwd diff-tree --no-commit-id --name-only -r HEAD) | Measure-Object).Count
+
+# Trivial-prefix check (Conventional Commits prefix before optional scope + colon)
+$prefix = $subject -replace '^([a-z]+)(\([^)]+\))?:.*$','$1'
+$trivial = @('meta','docs','style','chore')
+if ($trivial -contains $prefix) { exit 0 }
+if ($subject -match 'fix\s+typo') { exit 0 }
+
+# Threshold check
+$bodyLen = if ($body) { $body.Length } else { 0 }
+if ($bodyLen -le 200 -and $files -le 3) { exit 0 }
+
+# Substantive — emit JSON
+$msg = "Substantive commit detected on " + $cwd + ": ``" + $subject + "`` (" + $files + " files changed, body " + $bodyLen + " chars). Consider invoking session-handoff write-mode to update .tasks/NEXT_SESSION.md."
+
+@{
+    hookSpecificOutput = @{
+        hookEventName     = 'PostToolUse'
+        additionalContext = $msg
+    }
+    systemMessage  = 'session-handoff: substantive commit detected'
+    suppressOutput = $false
+} | ConvertTo-Json -Compress -Depth 5 | Write-Output