feat: support dot-path notation in Story.set and Story.get

[rohal12]

Summary

• Story.set('alma.view', 'home') now sets the view property on the $alma object instead of creating a flat variable named "alma.view"
• Story.get('alma.view') traverses nested properties, consistent with twee {$alma.view}
• Works in both string and batch object forms, including transient (%) variables

Closes #150

Test plan

• 15 new unit tests covering: nested get/set, deep paths, missing intermediates, array index access, transient dot-paths, batch form with mixed keys, error on non-object intermediate
• All 1136 existing tests pass — zero regressions
• tsc --noEmit clean

🤖 Generated with Claude Code

[github-actions]

Release preview: merging this PR will publish v0.44.0 (minor bump from v0.43.7)

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	77.9%	2856 / 3666
🔵	Statements	76.8%	3116 / 4057
🔵	Functions	69.47%	519 / 747
🔵	Branches	74.09%	1579 / 2131

File Coverage

File	Stmts	Branches	Functions	Lines	Uncovered Lines
Changed Files
src/story-api.ts	45.1%	38.46%	31.25%	46.19%	76, 102-104, 239, 268-276, 284-368, 381-459, 475-492, 519-524, 551-586

Generated in workflow #234 for commit ec0b2f3 by the Vitest Coverage Report Action