feat(sessions): play a custom sound file on run settle

[Direct-Launch]

What

Adds optional custom sound-file paths to the run-settle cue. Two settings, runSuccessSoundPath and runFailureSoundPath, take absolute OS paths; the file is read from disk and decoded at play time, falling back to the built-in 880/320 Hz tone when the path is empty, the file is missing, or it decodes to nothing. cueForOutcome stays pure, so the decision is testable without an AudioContext.

How I tested it

Real vault, Obsidian 1.13.7, desktop (Windows). Set an absolute success path in Agents settings, started a run from a gesture (send/Enter, which is what unlocks audio), and confirmed the configured file plays when the run settles — the built-in tone is not used. The change is additive: with no path set, the existing tone still plays.

Unit level: 13 targeted tests, and the two new file-branch tests were falsified first — forcing the branch to null turned exactly those two red and left the other eleven green.

The failure-path file was not separately exercised; it shares the same code path as success.

AI assistance: drafted by an AI agent (Smart Second Brain); reviewed and tested live by me.

Design

• cueForOutcome returns { kind: "default", frequency } | { kind: "file", path } | null — pure, so no AudioContext in tests.
• Decoded buffers and in-flight decodes are cached per path, so a repeated cue does not re-read from disk.
• Desktop only: readFileBytes returns null where there is no require, and the fields sit inside the existing isMobileUI guard.
• A missing or undecodable file degrades to the built-in tone — it never throws into the run bookkeeping.

Checklist

• check / format / lint pass locally — run via the npm equivalents (npx biome format, npx biome lint exit 0 across ./src ./test ./integration; svelte-check 0 errors / 0 warnings on both configs). bun is not available on the machine this was prepared on.
• test — all 13 new tests pass. The suite reports one failure, test/skills/shippedSkillHistory.test.ts, which is pre-existing and Windows-clone-only (core.autocrlf=true, no .gitattributes): it fails identically with this change reverted, and CI checks out LF.
• Tried the change in a real Obsidian vault (see above)
• I read CONTRIBUTING.md
• No provider, bundled skill or built-in tool added, and manifest.json is unchanged, so the docs site needs no update

Stacked on

Base is integrate/sidebar-2026-10-01, which carries the settle-cue fixes, so this diff is the sound-file change alone. CONTRIBUTING asks for PRs against main; this one is deliberately stacked, consistent with the existing #1 / #2 chain.

[Direct-Launch]

Consolidated into #1. Every commit here is included in feat/session-sidebar, which now targets main and carries the whole stack plus its real-vault test evidence. Closing this so there is one review surface.