fix: skip hook injection for non-interactive (sdk-cli) sessions

gistfile1.txt

# Bug: Caveman hooks fire on sdk-cli probe sessions

## Summary

Caveman's `SessionStart` and `UserPromptSubmit` hooks fire on every Claude
session, including non-interactive `sdk-cli` sessions spawned by external
tools. This causes those tools to receive caveman-mode responses instead of
the plain output they expect, silently breaking their functionality.

## How it was discovered

CodexBar (a macOS menu bar app that tracks Claude usage) shows the error:

> Could not parse Claude usage: Missing Current session.

Investigation traced the failure to CodexBar's probe mechanism. Every ~2
minutes, CodexBar spawns a Claude `sdk-cli` session with `cwd` set to
`~/Library/Application Support/CodexBar/ClaudeProbe` and sends the prompt
`"show usage\n\n"`. It then parses the response text using regex patterns
to extract current session token/cost data.

## Root cause

The `~/.claude/sessions/` directory confirmed the probe sessions have
`"entrypoint":"sdk-cli"` in their session files. Inspecting the probe
session's JSONL (`~/.claude/projects/-Users-hayden/<session-id>.jsonl`)
revealed the full injection chain:

**Attachment: `SessionStart:startup` hook**
```
CAVEMAN MODE ACTIVE — level: full
Respond terse like smart caveman...
```

**Attachment: `UserPromptSubmit` hook**
```
CAVEMAN MODE ACTIVE (full). Drop articles/filler/pleasantries/hedging...
```

**Attachment: `skill_listing`** (injected by UserPromptSubmit hook)
```
- caveman:caveman-stats: Show real token usage...
```

With the caveman ruleset and skill listing injected, Claude matched
`"show usage"` → "token usage/savings" → `caveman:caveman-stats` skill.
The `caveman-stats` hook then blocked the turn and injected stats output
instead of a response. CodexBar received:

> Hook blocked — stats injected directly. If nothing displayed, hook may
> not have fired correctly. Run `/caveman-stats` in the input instead.

This is unparseable by CodexBar's regex patterns → "Missing Current session."

## Affected tools

Any tool that spawns a Claude `sdk-cli` session hits this:

- CodexBar (usage monitoring)
- CI/CD pipelines using Claude SDK
- Automation scripts using `claude -p` or the Claude SDK
- Other IDE extensions or menu bar apps with Claude integrations

## Fix

Both hooks check `transcript_path` from stdin to identify the session, look
up the session file in `~/.claude/sessions/`, and read the `entrypoint`
field. If `entrypoint !== "cli"`, the hooks exit immediately without
injecting any caveman context.

`SessionStart` uses async stdin reading with a 200ms fallback timeout so it
does not block on sessions that may send no stdin data.

`UserPromptSubmit` returns `{}` (no-op) for non-interactive sessions.

Interactive `cli` sessions are unaffected.

## Testing

Verified with a synthetic sdk-cli session file:

```bash
# Create fake sdk-cli session
echo '{"pid":99999,"sessionId":"test-id","entrypoint":"sdk-cli",...}' \
  > ~/.claude/sessions/99999.json

# UserPromptSubmit hook returns {} (no caveman injection)
echo '{"transcript_path":".../test-id.jsonl","prompt":"show usage"}' \
  | node src/hooks/caveman-mode-tracker.js
# → {}

# Interactive session still gets caveman context
echo '{"transcript_path":".../real-session.jsonl","prompt":"show usage"}' \
  | node src/hooks/caveman-mode-tracker.js
# → {"hookSpecificOutput":{"additionalContext":"CAVEMAN MODE ACTIVE..."}}
```