fix: log every line of multi-line messages - #8

The daemon log truncates any message at its first newline and appends ....
Process error output is routinely multi-line, and the cause of a failure
usually appears after the first symptom of it, so the useful half of the
message was being discarded.

Each line of a message is now written as its own log line carrying the full
timestamp, level, and scope prefix. Neutralizing control characters per line
also makes the separate carriage-return truncation unnecessary.

Found while investigating a customer file sync failure, where the logged
error named a symptom and hid the cause:

[I] [sync.sync_IxTbhpj3] Beta connection failure: ... unable to copy agent
binary: unable to run SCP process: /bin/sh: -c: line 1: syntax error near
unexpected token `%a'...

/bin/sh: 1: Syntax error: Bad for loop variable
/bin/sh: 1: exec: /c/Program Files/Coder/bin/coder.exe: not found
scp: Connection closed

2026-08-12 23:08:21 [I] [sync.sync_IxTbhpj3] unable to run SCP process: /bin/sh: 1: Syntax error: Bad for loop variable...

2026-08-12 23:08:21 [I] [sync.sync_IxTbhpj3] unable to run SCP process: /bin/sh: 1: Syntax error: Bad for loop variable
2026-08-12 23:08:21 [I] [sync.sync_IxTbhpj3] /bin/sh: 1: exec: /c/Program Files/Coder/bin/coder.exe: not found
2026-08-12 23:08:21 [I] [sync.sync_IxTbhpj3] scp: Connection closed

Notes for the reviewer:

• Lines are written in a single Write call so a multi-line record stays
  contiguous when several goroutines log at once.
• pkg/agent has two pre-existing test failures on a clean main; they need
  a built agent bundle and are unrelated.
• The Commit Verification job will fail. These commits are not
  cryptographically signed, and re-signing them means rewriting history,
  which I am not permitted to do. Existing commits on this fork are also
  missing their sign-off. Happy for someone to re-sign before merge.

This PR was created by Coder Agents on behalf of @aslilac.