Fix last-msg and pr-url commands to use correct activity field names

[google-labs-jules]

Fixes issue #9.

• Both last-msg and pr-url commands in packages/cli/src/index.ts parsed Jules activity fields that were guessed and never verified.
• last-msg has been updated to extract text properly from lastMsg.planGenerated.plan.steps or lastMsg.userFeedbackRequired.message.
• pr-url has been updated to use session.outputs[].pullRequest.url fetched via getSession(id) instead of searching activities.

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PR created automatically by Jules for task 2705438157339577511 started by @GreyC

[google-labs-jules]

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[GreyC]

LGTM. Both fixes match the confirmed API structure from live session data:

• last-msg: correctly handles typed union fields (planGenerated.plan.steps, userFeedbackRequired.message) with JSON fallback
• pr-url: correctly reads from session.outputs[].pullRequest.url via getSession() instead of searching activities

Verified against real API responses from sessions 2705438157339577511 and 5813762652404726973.