Rename top-level `src` package to `mcp_ynab`

[pragprogrammer]

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The wheel installs a top-level src package into site-packages:

[tool.hatch.build.targets.wheel]
packages = ["src"]

[project.scripts]
mcp-ynab = "src.server:main"

Any other distribution that does the same thing collides with us in a shared
environment, and src is a common enough choice that this is a real risk rather
than a theoretical one. Whichever package installs last wins, and the failure
mode is confusing import errors far from the cause.

Scope

Smaller than it looks:

• The console script stays mcp-ynab, so anyone running uvx mcp-ynab or using
  a normal MCP client config is unaffected.
• It breaks python -m src.server, which is documented in CLAUDE.md and would
  become python -m mcp_ynab.server.
• The diff is wide but mechanical: src/ moves to mcp_ynab/, every
  from src.x import y is rewritten, plus tests and the entry point.

Deliberately kept out of #22 so the mcp 2.0.0 outage fix stayed reviewable. There
is a matching TODO in pyproject.toml.

Worth shipping as its own minor release.
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