fix(terminal): reduce default scrollback from 2500 to 1000 lines

[gregpriday]

Summary

• Reduces SCROLLBACK_DEFAULT from 2500 to 1000 lines, cutting per-terminal buffer memory by 60% and bringing Canopy in line with VS Code's default
• Lowers the agent scrollback maximum cap from 10,000 to 5,000 lines, reducing worst-case memory with 20 agent panels open from ~300MB to ~150MB
• Adds a store migration (007) so users on the old default are silently moved to the new one on next launch, without touching settings they've manually changed

Resolves #3278

Changes

• shared/config/scrollback.ts — SCROLLBACK_DEFAULT 2500 → 1000
• electron/store.ts — default store value updated to match
• src/utils/scrollbackConfig.ts — agent max cap 10,000 → 5,000; UI slider max updated
• electron/services/migrations/007-reduce-default-terminal-scrollback.ts — new migration that resets the stored value when it matches the old default
• src/components/Settings/TerminalSettingsTab.tsx — memory estimate and slider label reflect new bounds
• Tests updated across scrollback.test.ts, StoreMigrations.test.ts, and TerminalInstanceService.scrollback.test.ts

Testing

• npm run check passes clean (0 errors, warnings only from pre-existing patterns)
• Unit tests for the migration, scrollback config, and terminal instance service all pass
• Memory estimate in the settings UI correctly reflects the updated default and agent cap

[gregpriday]

Review status: Ready

• Cross-PR note: PR feat(terminal): reduce xterm.js scrollback for background terminals #3361 also modifies scrollback.ts and scrollback tests. Merge this PR first, then rebase feat(terminal): reduce xterm.js scrollback for background terminals #3361. SCROLLBACK_BACKGROUND in feat(terminal): reduce xterm.js scrollback for background terminals #3361 has been updated to 500 to remain meaningful after this PR's default reduction.