Add VS Code command to generate sprint report PDFs

[VeryComplexAndLongName]

Summary

• Second half of the sprint-report feature (add-sprint-report-pdf, PR Add sprint summary PDF report (standalone) #96, shipped the standalone-only half). New Command Palette command "OpenSpec UI: Generate Sprint Report (PDF)": reuses pickChangesForTimeline unchanged, prompts for a validated YYYY-MM-DD start/end date (no native VS Code date picker; this command needs a real user-specified range, unlike the comparison timeline's auto-derived one), calls buildSprintReport/renderSprintReportPdf from @openspec-ui/core directly (per ADR-0001 — no server/REST round-trip), then showSaveDialog -> workspace.fs.writeFile -> a confirmation message offering to open the PDF.
• Caught and fixed a real regression before it shipped: this is the first code path that actually executes renderSprintReportPdf from inside the bundled extension. That re-exposed the pdfkit/esbuild bundling hazard PR Add sprint summary PDF report (standalone) #96's CI caught — the createRequire-based workaround from that PR fixed extension activation, but silently stopped esbuild from inlining pdfkit into the bundle at all (confirmed: dist/extension.js dropped from 2.8MB to 635KB and left a real runtime require("pdfkit") call). Since vsce package --no-dependencies ships no node_modules, the command would have failed at runtime with "Cannot find module 'pdfkit'" the first time a real user ran it.
• Fix: reverted sprint-report-pdf.ts to a plain import PDFDocument from "pdfkit" (correct as-is for every plain-Node/ESM consumer — server, core's own tests) and instead fixed the bundling problem where it actually lives — the extension's own esbuild config — via an alias mapping "pdfkit" to its real CommonJS build file at bundle time (build-options.mjs), which genuinely inlines it and reads __filename instead of import.meta.url.

Verification

Rebuilt dist/extension.js and loaded it in plain Node with a stubbed vscode module: activation succeeds, and the bundle contains pdfkit's real CommonJS build inline (confirmed via ICC_PROFILE_PATH's pathToFileURL(__filename) form, not the broken import_meta2.url form from before the fix).

Test plan

• New tests in commands.test.ts for generateSprintReport: builds+saves+offers to open the PDF; no-op when no changes picked; no-op when either date prompt is dismissed; date validator rejects malformed input; no file written when the save dialog is dismissed; PDF not opened when the confirmation is dismissed; error surfaced when report generation fails
• showSaveDialog/workspace.fs.writeFile/env.openExternal stubs added to test-utils/vscode-mock.ts
• npm run typecheck, npm run lint (including lint:english), npm run test workspace-wide
• Rebuilt dist/extension.js and verified real pdfkit-CJS inlining + successful activation in plain Node (see above)
• openspec change validate --strict add-sprint-report-vscode-command

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