fix(messages): send parse=none on chat.update so links survive an edit (#106) (#120)

Closes #106, which you labelled `ready-for-pr`.

## What changed

`src/lib/slack-client.ts` — one parameter:

```ts
return this.request('chat.update', { channel, ts, text, parse: 'none' });
```

The comment above it records why it has to be explicit, since the value
looks redundant next to `chat.postMessage`: [the
docs](https://docs.slack.dev/reference/methods/chat.update) say `parse`
"Defaults to `client`, unlike `chat.postMessage`", and also that "If you
do not specify a value for this field, the original value set for the
message will be overwritten with the default" — so omitting it does not
inherit what the message was posted with.

## Verified on a real workspace

v0.9.0 from source against browser (xoxc/xoxd) auth, in a DM, same text
every time, reading `text` back with `conversations.history`:

| step | stored `text` |
|---|---|
| posted with `chat.postMessage` | `<https://example.com\|labelled
link>` |
| edited by v0.9.0 (no `parse`) | `&lt;https://example.com\|labelled
link&gt;` |
| edited with `parse: 'none'` | `<https://example.com\|labelled link>` |

Two details from the repro worth having on record:

- The corruption does not compound. Editing three more times leaves it
at `&lt;…&gt;` rather than escaping the `&`, so a message is broken once
and stays that way. It does not self-heal either — text posted as a link
never becomes a link again without a corrected edit.
- `<@U…>` mentions came through all three steps intact, so this is
specifically the `<url|label>` markup, not user references. `link_names`
also defaults to `none` on `chat.update` per the docs; I left it alone
since nothing in this repro needed it.

## Tests

`src/lib/slack-client.test.ts`: the existing `updateMessage` assertion
now includes `parse: 'none'` in the expected params, plus one case named
for the failure it prevents. The test double records `{method, params}`,
so it pins the wire call — which is where the bug was.

`bun test` 405 pass / 0 fail, `bun run type-check` clean. `pre-commit`
is not installed on this machine, so those hooks ran only in the form CI
runs them.

## Not covered

`chat.postMessage` is untouched: its `parse` default is already `none`,
and the repro confirms posted text arrives unescaped.

Co-authored-by: VibeXP Agent <hello@vibexp.io>