Strip copies in the substring filter, not via yank-handler

[OSadovy]

Problem

Rendered output and block-quoted replies carry a yank-handler text property
whose lambda strips properties (and the leading > ) at paste time. The
property rides on the characters, which has surprising reach:

• Once the text lands in any other buffer, copies from there still run
  agent-shell's lambda - for the rest of the session, in buffers unrelated to
  agent-shell.
• insert-for-yank is not just C-y: registers, rectangles, cua, xterm
  paste, and any third-party property-preserving insertion all hit the handler.
• The same M-w yields different content depending on destination: C-y
  gives hello, while the OS clipboard and isearch's C-s C-y keep the
  stored > hello. yank-excluded-properties is never consulted for handler
  text, so no setting affects any of this.

Change

Delete both yank-handler puts and finish the job in
agent-shell--filter-buffer-substring, which shell and viewport buffers
already install as filter-buffer-substring-function - the hook Emacs
provides for deciding what a copy of a buffer yields. Copies now come out
property-free, and one > per line is dropped from text tagged with a
private agent-shell-block-quote marker (set by agent-shell--block-quote
in place of the handler). The decision happens once, at copy time, in buffers
agent-shell owns; paste stays stock Emacs.

Behaviour

M-w, C-w, mouse copy, and registers out of agent-shell buffers paste the
same as before. Markdown-rendered blockquotes still keep their > on copy,
so the source round-trips. Two visible changes, both toward consistency:

• Copies of block-quoted replies are now identical everywhere. Previously the
  OS clipboard and isearch kept the > that yank removed.
• Strings from the standalone agent-shell-markdown-convert API, killed from
  non-agent-shell buffers, now paste with stock Emacs semantics. No in-tree
  caller does this. Happy to gate it behind a keyword if you'd rather keep
  the old behaviour there.

Context

Found while speech-enabling agent-shell with Emacspeak: its speech layer
inserts buffer text via insert-for-yank precisely to keep the text
properties it builds voices from, and the handler strips them. Any consumer
using insert-for-yank for property-preserving insertion hits the same wall,
and a buffer-local filter cannot break them.

Tests: the yank test is rewritten as filter tests - properties stripped,
> dropped from quoted replies, one level per line for nested quotes,
markdown blockquotes keep theirs. Full suite passes.

Checklist

• I agree to communicate (PR description and comments) with the author myself (not AI-generated).
• I've reviewed all code in PR myself and will vouch for its quality.
• I've read and followed the Contributing guidelines.
• I've filed a feature request/discussion for a new feature. (N/A - bug-ish cleanup, no new feature)
• I'm making visual changes, so I'm including screenshots so you can view and discuss. (N/A - no visual changes)
• I've added tests where applicable.
• I've updated documentation where necessary. (docstrings updated)
• I've run M-x checkdoc and M-x byte-compile-file.