chore: release 1.0.0

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Gives the port its own version and releases it as 1.0.0. Until now the plugin borrowed pstack-claude's number (0.9.57-copilot.N). VERSION now holds the port's semver, and upstream.json keeps the pinned pstack-claude commit and version separately.

Changes

• VERSION holds 1.0.0. tools/build.mjs reads it, rejects anything that isn't MAJOR.MINOR.PATCH, and stamps it into plugin.json and the marketplace manifest.
• tools/check.mjs checks the stamped version and that the description names the pinned upstream version.
• README shows 1.0.0, points version pinning at v1.0.0, and explains VS Code's "Allow reading file outside of workspace?" prompt on a session's first read of pstack's files.
• docs/maintaining.md says when to bump patch, minor, or major.

Verified:

• node tools/check.mjs: 20/20. A VERSION of 1.0 stops the build.
• The shipped plugin differs from the previous build only in its version field.
• In VS Code 1.140's agents window, on the Copilot target, against the previous build: the agent picker lists no pstack agents, the startup instructions reach the model, poteto-mode loads, and an unprompted multi-file question routes into pstack and changes no files.