feat: integrate session summary hook for end-of-session analytics

Problem

RTK tracks token savings via rtk gain, but users must manually run it to see their savings. There's no automatic visibility into the value RTK provides at the end of each Claude Code session.

Meanwhile, 4 open issues on anthropics/claude-code request exactly this kind of session analytics — and Anthropic hasn't shipped anything:

• #1109 - Usage Metrics Visibility for Max Subscribers
• #18550 - Automatic Cost Tracking and Reporting
• #16373 - Auto-spawn subagents with persistent session summaries
• #22625 - Per-Subagent Token Usage Tracking

Existing Solution

A community-built Session Summary hook already solves this. It's a Claude Code SessionEnd hook (bash) that displays 15 configurable analytics sections at every session exit:

• Duration, tools, errors, files, git diff, LOC, models, cache, cost, RTK savings, ratio, thinking, context...
• RTK integration: captures baseline at SessionStart (rtk gain snapshot), computes per-session delta at SessionEnd
• Fully toggleable: if RTK isn't installed, the section is simply hidden
• Single dependency: jq
• Open source: claude-code-ultimate-guide/examples/hooks

How RTK integration works

1. SessionStart hook (rtk-baseline.sh): runs rtk gain > /tmp/rtk-baseline-<key>.txt
2. SessionEnd hook (session-summary.sh): reads baseline, runs rtk gain again, computes delta
3. Displays per-session savings: commands count, tokens saved, percentage

Note: The hook currently uses rtk gain text output. The --format json flag could enable more precise parsing.

Proposal

Option A: rtk install-hooks (recommended quick win)

New command that installs the SessionStart + SessionEnd hooks into ~/.claude/hooks/ and patches settings.json:

rtk install-hooks              # Install session summary hooks
rtk install-hooks --uninstall  # Remove them

Pros: One-command setup, leverages existing battle-tested hook code, minimal Rust changes
Cons: Bash dependency, external hook maintenance

Option B: rtk session-summary (native Rust)

RTK embeds the session summary logic natively. Pure Rust, no bash/jq dependency. Could hook into Claude Code's SessionEnd event.

Pros: No external dependencies, faster, tighter integration
Cons: Significant development effort, needs to replicate 15 sections of analytics

Option C: Documentation + reference (immediate)

Add a "Companion Tools" section to the README documenting the session summary hook as a recommended companion for automatic RTK savings visibility.

Pros: Zero code changes, immediate value, community attribution
Cons: Users must install manually, no tight integration

Option D: Bundle hook in repo

Copy the hook scripts into a hooks/ directory in the RTK repo, maintained in sync with upstream.

Pros: Single repo, discoverable
Cons: Sync maintenance burden, potential version drift

Recommendation

Phase 1 (now): Option C — document the hook in README as companion tool
Phase 2 (next): Option A — rtk install-hooks command for one-click setup
Phase 3 (roadmap): Option B — native Rust implementation for zero-dependency experience

References

• Session Summary hook: https://github.com/FlorianBruniaux/claude-code-ultimate-guide/blob/main/examples/hooks/bash/session-summary.sh
• RTK baseline hook: https://github.com/FlorianBruniaux/claude-code-ultimate-guide/blob/main/examples/hooks/bash/rtk-baseline.sh
• Config CLI: https://github.com/FlorianBruniaux/claude-code-ultimate-guide/blob/main/examples/hooks/bash/session-summary-config.sh
• Hook documentation: https://github.com/FlorianBruniaux/claude-code-ultimate-guide/blob/main/examples/hooks/README.md#session-summarysh-v3

Start by reviewing the referenced upstream hook files, the README, and the Claude Code settings.json hook format; run rtk gain to understand the current output and baseline flow. Clarify which proposal is in scope, then define completion as either documenting the companion hook, installing it reliably, or implementing the native session summary, with tests or verification for that chosen path.