feat(api): expose generated episode transcripts

[chhoumann]

Expose generated PodNotes transcript notes through the public plugin API so QuickAdd, Templater, and other workflow plugins can read transcript text from app.plugins.plugins.podnotes.api.

This adds api.transcript as a convenience async getter and api.getTranscript(episode?) for current or explicit episode lookup. Both read the already-generated transcript note using the same configured transcript path logic as the transcription writer; they do not trigger transcription, call OpenAI, or create new transcript files.

Docs now include a concrete QuickAdd-style macro example that builds an AI prompt from the PodNotes transcript. The transcript docs also clarify that the transcript template placeholder is {{transcript}}.

Validation performed:

• Real Obsidian isolated worktree vault: reproduced missing transcript API on current master-equivalent bundle; after implementation reloaded PodNotes and verified api.transcript, api.getTranscript(), and api.getTranscript(explicitEpisode) return the generated transcript note, with missing transcript returning null and dev:errors clean.
• fnm exec --using 22 npm run lint
• fnm exec --using 22 npm run format:check
• fnm exec --using 22 npm run typecheck
• fnm exec --using 22 npm run build
• fnm exec --using 22 npm run test
• PATH=/tmp/podnotes-codex-105-mkdocs/bin:$PATH fnm exec --using 22 npm run docs:build

Adversarial review covered correctness/runtime behavior, public API ergonomics, security/privacy/cost, and maintainability/test quality. Follow-up fixes added explicit-episode API test coverage and corrected the transcript placeholder docs.

Closes #105

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