[Agent Fix]: Old tail anchor not found after old head

[kamranjon]

PR Summary: Fix anchored edit tail anchor search

Bug: edit with [upto] always fails with old tail anchor not found after old head, blocking large anchored replacements.

Root Cause: In agent_edit_find_old_span(), the tail string includes a leading \n (from [upto]\n), but the file after the head match has no such newline — so the tail needle never matches.

Fix: Strip leading \n/\r from the tail before searching. The head already ends with that newline.

Changes (ds4_agent.c lines 5871–5879):

• Added a while-loop to remove leading \n/\r from tail / tail_len.
• Works for both Unix (\n) and Windows (\r\n) line endings.
• Handles both the forcer-injected \n and model-written \n after [upto].
• The agent_span_has_nonspace guard (moved after the strip) still rejects empty/whitespace-only tails.
• match_len remains correct because the head's trailing newline is counted in head_len.

Testing: Compiles cleanly with -Wall -Wextra, all edge cases verified. [upto] edits actually work. I had to go back to a sha from 2 weeks ago to find an agent that could properly use the edit tool again to make this change: d447bdb - but tested with newest build and agent is able to edit reliably again!