fix(agent/agentproc): read process info before output to prevent TOCTOU

[mafredri]

handleProcessOutput read proc.output() then proc.info() using separate locks (buf.mu vs proc.mu). Between the two reads, the exit goroutine could finish I/O and set running=false, pairing a stale buffer snapshot with final process status.

On Windows CI this caused TestProcessLifecycle/OutputExceedsBuffer to flake: the buffer was captured mid-write (OmittedBytes=0) while info reported the process as exited.

Swap the read order: info() first, then output(). The exit goroutine completes cmd.Wait() (draining all pipe data) before setting running=false, so Running=false guarantees the subsequent output read reflects the final buffer state.

Proof: dlv-controlled TOCTOU reproduction

Used dlv to freeze execution between the two reads and mutate process state, proving the race under controlled conditions.

Buggy order (output-first, breakpoint between reads):

=> 204:    info := proc.info()   // output() already returned

(dlv) print truncated
  OmittedBytes: 0          // stale: captured before mutation
(dlv) print proc.running
  true
(dlv) print proc.buf.totalBytes
  96

# Simulate exit goroutine via dlv:
(dlv) call proc.buf.Write(...)
(dlv) set proc.running = false
(dlv) print proc.buf.totalBytes
  500

# Continue -> info() returns Running=false
# Response: Running=false, OmittedBytes=0  <- TOCTOU proven

Fixed order (info-first, breakpoint between reads):

=> 217:    output, truncated := proc.output()  // info() already returned

(dlv) print info
  Running: true            // captured before mutation

# Same mutation via dlv
# Continue -> output() returns fresh buffer
# Response: Running=true, OmittedBytes=372  <- safe, client polls again

Closes https://linear.app/codercom/issue/CODAGT-399

🤖 This PR was created with the help of Coder Agents, and will be reviewed by a human. 🏂🏻