Show Welcome screen on extension activation, not on session login

[claude]

Summary

The Welcome screen currently opens when a user logs into a session, which is too late to
serve as an onboarding guide for new users. Showing it at extension install or activation
time — and expanding its content to cover how to get connected to GemStone — would make
it genuinely useful as a "getting started" resource.

Acceptance criteria

• The Welcome screen opens the first time the extension is activated (install or VS Code
  startup after install), not on session login.
• The Welcome content includes enough getting-started material for a brand-new user to
  understand how to download GemStone and establish their first session.
• The existing "Getting Started…" mortar-board entry and command palette option continue
  to work so users can reopen it on demand.
• The "show only once per machine" behaviour is preserved.

Implementation notes

One approach could be to move the trigger from the session-login path to the extension
activate() entry point, gating on the same "seen on this machine" flag already used for
the once-per-machine logic. The Welcome content itself would need a new section — possibly
a step-by-step guide covering GemStone download, stone setup, and first connection.

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Migrated from GitLab: GemStone/grail#49

[ericwinger]

Closing — this shipped. All four acceptance criteria are met in main.

Opens on first activation, not on session login. activate() calls maybeOpenGettingStarted(context) (client/src/extension.ts), and the extension declares onStartupFinished in activationEvents. The trigger is deliberately keyed off activation rather than a view's visibility, so marking a view "visibility": "collapsed" can't silently disable onboarding.

Content covers getting connected from scratch. The gemstoneGettingStarted walkthrough is titled "Get Started with GemStone", described as "Download GemStone, create a database, connect, and explore Smalltalk expressions in Jasper." Its first step, resources/walkthrough/setup.md, covers downloading a GemStone/S 64 version, creating a database, starting the stone and NetLDI, and the DataCurator login — with Quick Setup as one click plus a by-hand path via the Versions and Databases views. Then connect.md, workspace.md, displayIt.md, inspectIt.md, enhancedInspector.md, and a reopen step.

Reopenable on demand. gemstone.openWalkthrough carries the $(mortar-board) icon, sits at navigation@0 on the Versions view title, and is in the palette under the GemStone category. Its title is now "Get Started with GemStone" rather than the older "Getting Started…" — same entry, reworded. gemstone.resetGettingStarted makes it auto-open again on the next startup.

Once per machine, preserved. Gated on the gemstone.hasSeenGettingStarted globalState key, which is set before the walkthrough opens so a re-entrant call can't double-open it.

History: the walkthrough replaced the auto-opened welcome workspace in 551f5de (#126), the trigger moved off session login to first view reveal in fd41594, and then to activation in d6ce56a — which is what this issue asked for.