Previously saved conversations won't load

### Checks

- [x] I have searched [github.com/aws/amazon-q-developer-cli/issues](https://github.com/aws/amazon-q-developer-cli/issues?q=) and there are no duplicates of my issue
- [x] I have run `q doctor` in the affected terminal session
- [x] I have run `q restart` and replicated the issue again

### Operating system

macOS 15.6.0 (24G84)

### Expected behaviour

Expected to load the conversation saved with /save without errors

### Actual behaviour

Trying to import three previously saved conversations yields the error:
Failed to import from ./conversation.md: missing field `timestamp` at line 21 column 7
Always the same field and location (21,7) - however, after manually adding a synthetic timestamp to the specified location the error moves forward in the file.

### Steps to reproduce

Save from a pre 1.13.2 version of Q Chat CLI then reload with current version
Missing timestamp error should happen
Adding timestamp manually fixes error though this is time consuming and error prone

### Environment

```yaml

[q-details]
version = "1.13.2"
hash = "36e70d1cad4465a9ea479bb7f10d7973b717b4f7"
date = "2025-08-08T20:16:09.268082Z (2d ago)"

[system-info]
os = "macOS 15.6.0 (24G84)"
chip = "Apple M2"
total-cores = 8
memory = "16.00 GB"

[environment]
cwd = "/Users/USER/Work/"
cli-path = "/Users/USER/Work/"
install-method = "brew"

[env-vars]
PATH = "/opt/homebrew/bin:/opt/homebrew/sbin:/usr/local/bin:/System/Cryptexes/App/usr/bin:/usr/bin:/bin:/usr/sbin:/sbin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/local/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/bin:/var/run/com.apple.security.cryptexd/codex.system/bootstrap/usr/appleinternal/bin:/Library/Apple/usr/bin:/usr/local/go/bin:/Applications/iTerm.app/Contents/Resources/utilities:/Users/USER/.local/bin"
QTERM_SESSION_ID = "edb06f0213ff41a1bc4253889fb3b173"
Q_SET_PARENT_CHECK = "1"
Q_TERM = "1.13.2"
SHELL = "/bin/zsh"
TERM = "xterm-256color"
__CFBundleIdentifier = "com.googlecode.iterm2"
```