fix(codex): read full first line for session validation

`readFirstLine` allocated a fixed 16 KB buffer, but Codex CLI 0.128+
embeds the entire base_instructions / system prompt in the
`session_meta` line, pushing it past 20 KB. When the buffer doesn't
catch a newline, `isValidCodexSession` rejects the session, so every
recent Codex session is silently excluded from totals.

Switch to a streaming readline read so the first line is captured
regardless of length, and add a regression test that creates a
40 KB session_meta payload.

Locally, this changes my 30-day Codex total from €267 (only ~half
of sessions parsed) to €878 (all sessions parsed).

src/providers/codex.ts

@@ -1,4 +1,6 @@
-import { readdir, stat, open } from 'fs/promises'
+import { readdir, stat } from 'fs/promises'
+import { createReadStream } from 'fs'
+import { createInterface } from 'readline'
 import { basename, join } from 'path'
 import { homedir } from 'os'
 
@@ -70,21 +72,29 @@ function sanitizeProject(cwd: string): string {
 }
 
 async function readFirstLine(filePath: string): Promise<CodexEntry | null> {
-  let fh
+  // Codex CLI 0.128+ writes a session_meta line that can exceed 20 KB because
+  // it embeds the full base_instructions / system prompt. A fixed-size buffer
+  // would miss the trailing newline and reject the session as invalid.
+  // Stream the file via readline to read the first line regardless of length.
+  const stream = createReadStream(filePath, { encoding: 'utf-8' })
+  const rl = createInterface({ input: stream, crlfDelay: Infinity })
+  let firstLine: string | undefined
   try {
-    fh = await open(filePath, 'r')
-    const buf = Buffer.alloc(16384)
-    const { bytesRead } = await fh.read(buf, 0, 16384, 0)
-    if (bytesRead === 0) return null
-    const text = buf.toString('utf-8', 0, bytesRead)
-    const nl = text.indexOf('\n')
-    const line = nl >= 0 ? text.slice(0, nl) : text
-    if (!line.trim()) return null
-    return JSON.parse(line) as CodexEntry
+    for await (const line of rl) {
+      firstLine = line
+      break
+    }
   } catch {
     return null
   } finally {
-    await fh?.close()
+    rl.close()
+    stream.destroy()
+  }
+  if (!firstLine || !firstLine.trim()) return null
+  try {
+    return JSON.parse(firstLine) as CodexEntry
+  } catch {
+    return null
   }
 }
 

tests/providers/codex.test.ts

@@ -123,6 +123,32 @@ describe('codex provider - session discovery', () => {
     expect(sessions).toHaveLength(1)
   })
 
+  it('accepts session_meta lines larger than 16 KB (Codex CLI 0.128+)', async () => {
+    // Codex CLI 0.128+ embeds the full base_instructions / system prompt in the
+    // first session_meta line, often pushing it past 20 KB. Regression guard
+    // against a fixed-size buffer in readFirstLine.
+    const bigPayload = JSON.stringify({
+      type: 'session_meta',
+      timestamp: '2026-05-02T00:00:00Z',
+      payload: {
+        cwd: '/Users/test/big',
+        originator: 'codex-tui',
+        session_id: 'sess-big',
+        model: 'gpt-5.5',
+        base_instructions: { text: 'x'.repeat(40_000) },
+      },
+    })
+    await writeSession(tmpDir, '2026-05-02', 'rollout-big.jsonl', [
+      bigPayload,
+      tokenCount({ last: { input: 100, output: 50 }, total: { total: 150 } }),
+    ])
+
+    const provider = createCodexProvider(tmpDir)
+    const sessions = await provider.discoverSessions()
+    expect(sessions).toHaveLength(1)
+    expect(sessions[0]!.path).toContain('rollout-big.jsonl')
+  })
+
   it('skips files without codex session_meta', async () => {
     const [year, month, day] = '2026-04-14'.split('-')
     const sessionDir = join(tmpDir, 'sessions', year!, month!, day!)