feat(sidenote): self-contained capture + checkbox lifecycle; ignore the parking file

.gitignore

@@ -0,0 +1,2 @@
+# Personal sidenote parking file (see /sidenote) — never committed
+.claude/sidenotes.md

README.md

@@ -198,15 +198,15 @@ Then `/scalable` (tests the current direction) or `/scalable <a specific decisio
 
 You're mid-way through a big task and a passing thought hits — *"the retry logic probably has the same bug", "remind me to check the pricing later"* — something you don't want to forget. Drop it in the chat as-is and the agent reads it as a new request: it rushes the current task to "get to" your note, and the big task pays for it.
 
-[`commands/sidenote.md`](./commands/sidenote.md) reframes that thought as a **parked later-task, not a now-task**. The agent logs it verbatim to `.claude/sidenotes.md` (durable across compaction and session end), gives a one-line ack, and resumes *exactly* where it was — same scope, same pace, no cutting corners to reach the note. A bare `/sidenote` flushes the list back to you.
+[`commands/sidenote.md`](./commands/sidenote.md) reframes that thought as a **parked later-task, not a now-task**. The agent logs it verbatim to `.claude/sidenotes.md` (durable across compaction and session end) — with a one-line anchor (task in progress, the file it points at, branch) so it still makes sense when another session or agent reads it cold — gives a one-line ack, and resumes *exactly* where it was — same scope, same pace, no cutting corners to reach the note. A bare `/sidenote` flushes the open list back to you.
 
 ```bash
 mkdir -p ~/.claude/commands
 cp commands/sidenote.md ~/.claude/commands/      # personal, all projects
 # or: .claude/commands/  for one project
 ```
 
-Then `/sidenote <the thought>` to park one, or `/sidenote` to see what's parked. The notes land in `.claude/sidenotes.md` — add it to `.gitignore` if you'd rather not track it.
+Then `/sidenote <the thought>` to park one, or `/sidenote` to see what's parked. The notes land in `.claude/sidenotes.md`, a personal parking file kept out of git (this repo's `.gitignore` already excludes it) — so it's per-checkout scratch, not shared history.
 
 > **No slash commands?** For agents that read a rule file but have no `/` commands (Cursor, Codex, Copilot…), the same contract works as a plain-text convention: prefix the message with `SIDENOTE:` and the agent parks it instead of acting. Add one line to your rule file so it's honored reliably — see [`commands/sidenote.md`](./commands/sidenote.md) for the exact contract.
 

commands/sidenote.md

@@ -13,9 +13,17 @@ Park this thought — **$ARGUMENTS** — as a sidenote. A sidenote is a *later-t
 
 **Persist it so it survives this session** (context gets compacted; memory doesn't count as "logged"):
 
-1. Append one bullet to `.claude/sidenotes.md` at the project root — create the file (and `.claude/`) if missing.
-2. Format: `- [<today's date, YYYY-MM-DD>] <the thought, captured verbatim>`. Don't paraphrase or "improve" it — capture what was said.
+1. Append one entry to `.claude/sidenotes.md` at the project root — create the file (and `.claude/`) if missing.
+2. Format — an open checkbox, the verbatim thought, then a one-line **anchor** so the note still makes sense when another session or agent reads it cold (the verbatim thought alone is full of "this / here / it" that dangles without the moment that produced it):
+
+   ```
+   - [ ] [<today's date, YYYY-MM-DD>] <the thought, captured verbatim>
+     ↳ <task you were mid-way through> · <file/area the note points at, if "this"/"here" resolves to one> · <branch>@<short-sha>
+   ```
+
+   - **Thought — verbatim.** Don't paraphrase or "improve" it; capture what was said.
+   - **Anchor — coordinates you already hold, not a research task.** Fill it only from what's already in front of you: the task in progress, the file/area currently in focus, the current branch and short SHA (already in your session's git context). **Do not open files, grep, or investigate the note's subject to complete it** — that would break the park contract. Any field you don't already know, omit. The anchor records where you were standing; it never sends you looking.
 
 Then: **one-line acknowledgement** (e.g. `Parked in .claude/sidenotes.md — continuing.`) and **resume exactly where you left off**, as if the note had never arrived.
 
-**If `$ARGUMENTS` is empty** (a bare `/sidenote`), there's nothing to park — treat it as a *flush*: read `.claude/sidenotes.md` and list the currently-open sidenotes back to me so I can decide what to pick up. Change nothing, act on nothing, then resume.
+**If `$ARGUMENTS` is empty** (a bare `/sidenote`), there's nothing to park — treat it as a *flush*: read `.claude/sidenotes.md` and list the **open** entries back to me so I can decide what to pick up. Open means *not checked off* — every entry except the ones marked done (`- [x]`), so older bullets without a checkbox still count. Change nothing, act on nothing, then resume. (Checking an item off to `- [x]` happens when it's actually handled — not part of parking or flushing.)