Update main.go and cli API for signal-cancellable context (task 352/main)

main.go: set up signal.NotifyContext(SIGINT, SIGTERM) so long-running
operations (fzf, external editors) terminate gracefully on interrupts.
Call cli.New(ctx) explicitly and pass the context to c.Run(ctx, argv),
matching the task spec and allowing the context to flow through all
store/git/crypto operations.

cli: expose New(ctx) and Run(ctx, argv) as the public API; remove the
package-level Run() helper that created its own context.Background().

Verified: mage build produces ./bin/geheim; PIN=test ./bin/geheim version
prints "> geheim v0.4.0".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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