perf: lazy-load token burden command

[infectiousstupidity]

Fixes #33.

Alternative implementation to #34.

Problem

pi-token-burden currently loads the full command dependency graph when the extension itself is loaded, even when /token-burden is never used.

This includes relatively expensive modules such as the tokenizer, report UI, skill handling, and extension-analysis code, adding unnecessary cost to normal Pi startup.

Change

Keep the extension entrypoint lightweight and move the /token-burden implementation into runTokenBurden.ts.

index.ts now only:

1. registers /token-burden
2. dynamically imports runTokenBurden when the command is invoked
3. delegates execution to it

This creates one explicit lazy-loading boundary so future command-only dependencies naturally remain off the startup path.

Tests verify that the command module is not evaluated during extension registration, then loads and delegates correctly when /token-burden is invoked. The existing active-tool/envelope behavior test is retained with the extracted command implementation.

Validation

pnpm run check passes in a clean Linux checkout on Node 22, including all 213 unit tests.

Scope

This PR intentionally contains only the startup lazy-loading change. It does not include the other performance work from my fork.