fix(cli): prevent TypeError crash in ag read on non-text content (#4684)

[OneStepAt4time]

Summary

Fixes #4684 — ag read crashes with TypeError when session messages contain non-text content.

Problem

ag read \u003csession-id\u003e crashed with:

TypeError: Cannot read properties of undefined (reading 'split')
    at handleRead (dist/commands/read.js:48:31)

This happened when the /read API returned messages with undefined text and content fields, causing content.split('\n') to fail.

Fix

• Added null-safety check before content.split() in handleRead
• If content is not a string, print [non-text content] placeholder and continue
• Reordered variable declarations to ensure prefix is defined before the check

Tests

• Added src/__tests__/commands/read.test.ts with 3 tests:
  • resolveSessionId returns full UUID as-is
  • resolveSessionId resolves prefix to full UUID
  • resolveSessionId returns null for ambiguous prefix

Verification

• npx tsc --noEmit ✅
• npm run build ✅
• npx vitest run src/__tests__/commands/read.test.ts ✅ (3/3 pass)
• File size check: read.ts unchanged in size ✅

Residual Risk

• The fix handles undefined content gracefully but does not address why the API returns undefined fields. If the API contract changes, this may need revisiting.
• No integration test against live API — unit tests only.

[aegis-gh-agent]

✅ Approved.

Fix is correct — the null-safety guard on msg.text ?? msg.content and the string normalization before .split() will prevent the TypeError crash.

Two minor notes (non-blocking):

1. The typeof content !== 'string' check after the ternary is dead code — content is guaranteed to be a string at that point. It doesn't affect correctness, but could be removed in a follow-up.
2. The 3 new tests cover resolveSessionId but not the actual crash scenario (handleRead with non-text/undefined content). Consider adding a test for the non-string raw branch in handleRead to lock in the fix.

CI all green. Targeting develop. Good to merge.