Add Breadcrumbs feature

[v8vito]

Breadcrumb trails capture an ordered path through the code. Each crumb stores the target file/range, captured snippet, timestamp, and optional note. Trails live alongside regular notes in memory, load on activation from .security-notes-breadcrumbs.json, and auto-save whenever the store changes or on deactivate. (Paths are stored relative so the data travels cleanly inside the workspace.)

All breadcrumb data is local-only today; remote sync comes later.

Commands Added

• security-notes.breadcrumbs.createTrail – prompt for a trail name/description and set it active.
• security-notes.breadcrumbs.selectTrail – quick-pick to switch the active trail.
• security-notes.breadcrumbs.addCrumb – capture the current editor selection (or line) plus an optional note, append it to the active trail, and push the update through the store so persistence kicks in.
• security-notes.breadcrumbs.removeCrumb / editCrumbNote – quick-pick helpers for maintenance.
• security-notes.breadcrumbs.showTrailDiagram – reveals the Breadcrumbs view in the activity bar.
• security-notes.breadcrumbs.exportTrail – exports the active trail to a Markdown report summarizing every crumb with code fences and metadata.

Webview UI

• Breadcrumbs appear in their own activity-bar view. The sidebar shows an accordion list of crumbs (“Step N · file:line”) with note previews; expanding a crumb reveals the snippet and lets you click it to jump back into the editor. Header buttons are now “New trail” and “Export”, matching the command set.

Persistence

• The store serializes trails to .security-notes-breadcrumbs.json (configurable via security-notes.breadcrumbs.localDatabase). On activation we load the file and hydrate the store; every subsequent change triggers an auto-save. When you export, the Markdown file is written wherever you choose, independent of the store.