feat(gptel-tools): wire web_fetch as a local tool

Fourth ADOPT entry from `docs/design/gptel-tools-shortlist.org'.
Lets gptel pull a URL into the conversation so the model can read
docs / current API shapes / etc. without me copy-pasting.

Shape:

- URL must be `http://' or `https://' (file://, ftp://, javascript:,
  scheme-less, etc. are rejected at the validator).
- HTML responses go through `pandoc -f html -t plain' so the model
  gets a reading shape that isn't full of markup; falls back to
  `w3m -dump -T text/html' if pandoc isn't on PATH; signals
  `user-error' if neither is.  Pass `raw=t' to skip stripping.
- Output capped at 200KB by default, hard cap 1MB; `max_bytes'
  argument lets the caller pick a lower cap.  Truncation reported
  inline.
- 4xx / 5xx response codes signal `error' with the code -- the
  alternative is returning an error page body, which the model
  would treat as content.

`:confirm t' on the tool because every call is a real outbound
network request.  The tool's description warns that URLs go
wherever the user-agent points, including internal networks if
that's what the URL names.

`tests/test-gptel-tools-web-fetch.el' -- 20 tests across Normal /
Boundary / Error.  URL validator covers http / https / non-string
/ empty / non-http schemes.  `--effective-max-bytes' covers default
/ low-clamp / hard-cap / passthrough.  Truncate helper covers
under-cap / at-cap / over-cap with the marker.  HTML stripper runs
against real pandoc / w3m (both installed in dev env, neither
should mangle simple markup).  Orchestrator stubs
`cj/gptel-web-fetch--retrieve' via `cl-letf' to cover normal /
raw / 4xx / 5xx / oversize / bad-scheme paths.

Wired into `cj/gptel-local-tool-features' so gptel exposes the
tool on next restart.

Note: `call-process-region' invocation flattened to a single
`with-temp-buffer' with DELETE=t -- the initial draft nested a
second temp buffer and routed output to the inner one, which got
killed before `buffer-string' on the outer ran.  Test caught it.

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