[eatmoreduck]

Motivation & Context

A plugin that registers a preset at runtime (ctx.agentPresets.register({ id: 'aitoearn', ... })) and also ships the old-form preset directory ($DSH_HOME/.agent-presets/aitoearn/) makes every desktop startup abort: migrateLegacyAgentPresets() converts the directory into an inline preset-aitoearn row in cordis.patch.yml without knowing the runtime registration exists, the registry rejects the duplicate id (error agent-preset: Error: Duplicate agent preset: aitoearn), the row is required, and startup fails. Relaunching cannot clear it — the migration's existingIds check keeps the row in place forever. Reported in #814 with a manual workaround (hand-adding disabled: true to the generated row), which this PR automates.

Description & Review Guide

• What are the major changes? Three pieces: extractDuplicateAgentPresetId() in harness-runtime.ts reads the duplicate id from the latest stderr attempt logs (same shape as the existing extractDuplicateLoaderEntryId / extractSlotConflictName); disableMigratedLegacyPreset() in legacy-preset-migration.ts locates that one row inside the # dsh-desktop legacy presets block and inserts disabled: true at the entry's field indent (both the - id: built-in shape and the - insert: / - id: plugin shape), atomically; the plugin-recovery loop in index.ts checks this before blaming plugins, auto-disables at most once per id per session, and relaunches via launchWithFreshEvidence().
• What is the impact of the changes? The startup loop self-heals: the disabled row neither mounts nor joins the startup audit (the registry's auditRows skips entry.disabled), while existingIds still matches the retained preset-<id> line so the migration never rewrites or duplicates it. The plugin's own runtime registration stays the single live preset — the legacy copy steps aside instead of the user's plugin being quarantined. Rows outside the migration block are never touched, already-disabled rows are left alone, and a non-matching or missing row leaves the file unchanged so the existing recovery flow takes over.
• What do you want reviewers to focus on? The row-indent walk in disableMigratedLegacyPreset (entry fields end at the next - row at the entry indent) and the one-shot-per-id session set, which prevents a relaunch loop when the duplicate comes from something other than a migrated row.

The tests cover: id extraction from attempt logs (stderr-only, latest attempt); disabling only the named row in both shapes; idempotency (no second disabled:); and rows outside the block being ignored. Migration and recovery suites pass (32); the 39 office/ppt/docx template failures reproduce identically on a clean checkout of main and are unrelated.

Fixes #814