README: rewrite, and fix the install it documented

The old one had drifted into a wall of prose that described the project to
someone who already knew it. Rewritten around what a reader actually needs:
what this is, what it needs, how to run it, the keys, and what is in it --
with the logo up top and the sharp edges at the bottom instead of a wall of
disclaimers at the top.

Corrected while checking every claim rather than trusting it:

- pyproject depended on 'ida-codemode-mcp' from '../ida-codemode-mcp'. The
  package is 'ida-codemode' and the repo is '../ida-codemode', so 'uv sync'
  -- the documented install -- could not have worked for anyone. Fixed, and
  uv.lock regenerated (it now resolves, 39 packages).
- test counts were 257/733, they are 302/788
- the keys table is generated from the real BINDINGS: c/d/a/p/u, o/O/B, s for
  split, ctrl+t for structs, " for strings, all verified against app.py
- dropped the ida-pro-mcp/worker-era architecture section, which described
  files that no longer exist, and replaced it with the current layout
- links docs/CODEMODE_UPSTREAM.md and .fastfeedback/SPEED.md, which did not
  exist when the README was last touched

Every path and doc link in it is checked to exist.

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