[Bug]: /speckit-converge is not idempotent — re-runs append duplicate remediation tasks

## Version

spec-kit v1.0.1 (`.claude/skills/speckit-converge/SKILL.md`)

## Description

`/speckit-converge` only checks that `tasks.md` exists before assessing the codebase and appending remediation tasks. It keeps no persisted evidence that `/speckit-implement` actually ran against the *current* task list, and it does not map its findings onto existing **unchecked** tasks before appending.

Two consequences:

1. **Invoked early** (before implement has run, or after a partial implementation), it appends remediation tasks for work that is already represented by existing unchecked tasks — now under new IDs, splitting traceability for the same work across two task entries.
2. **Invoked twice** (e.g. converge → partial fix → converge again), gaps found both times are appended both times.

## Expected behavior

- Converge verifies prerequisite state (an implementation checkpoint, or at minimum warns loudly when `tasks.md` contains unchecked tasks that overlap its findings).
- Before appending, each finding is compared against existing unchecked tasks; covered work is reused/updated rather than duplicated, and only genuinely new gaps get new IDs.

## Context

Found while upgrading a template fleet from 0.5.0 to v1.0.1 and reviewing the newly vendored command. Related in spirit to the duplicate-detection work in #2968, but on the tasks.md side rather than GitHub issues.

Start by reading .claude/skills/speckit-converge/SKILL.md and inspect how it reads tasks.md, handles unchecked tasks, and appends remediation tasks. Compare that flow with /speckit-implement and the duplicate-detection work in #2968, then reproduce converge twice. Done means prerequisite state is checked or warned about, existing unchecked work is reused or updated, and only new gaps receive new IDs.

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