Remove plenary.nvim from the e2e harness once neotest drops it upstream

[rcasia]

Context

Follow-up from #317 / #330.

neotest-java's own code no longer depends on plenary.nvim (archived/unmaintained) — build_tool/launcher.lua now uses vim.system and util/detect_project_type.lua reuses util/dir_scan.lua's vim.uv.fs_scandir walk. The unit/integration test harness (scripts/minimal_init.lua) also runs plenary-free.

One transitive use remains, and it is neotest's, not ours — tracked upstream in nvim-neotest/neotest#611:

• neotest.lib.file, neotest.lib.positions, and neotest.lib.subprocess internally require plenary.path / plenary.filetype (verified against neotest @ 7bef09d1).
• Our e2e harness builds a user-like environment by hand (nvim --headless --noplugin -u tests/testrc.vim) instead of using a plugin manager that would resolve neotest's dependencies for it.
• Therefore Makefile (the deps/plenary.nvim target, commented as of refactor: replace plenary.nvim with native vim.system/vim.uv (#317) #330) and tests/testrc.vim still clone/append plenary.nvim. An attempt to remove it in refactor: replace plenary.nvim with native vim.system/vim.uv (#317) #330 made the e2e jobs hang at "Running tests via Neotest..." (neotest's require('plenary.path') fails, run.lua polls for results that never arrive, job dies at the 5-minute timeout) — fixed in 8bdf2ab by restoring it with explanatory comments.

Task — when (not if) neotest drops plenary

Once nvim-neotest/neotest no longer requires plenary.* internally (track: nvim-neotest/neotest — search their repo for plenary; note neotest itself may need its own de-archiving/removal PR):

1. Remove the deps/plenary.nvim target and its install: dependency from Makefile.
2. Remove the set runtimepath+=./deps/plenary.nvim line (and comment) from tests/testrc.vim.
3. Remove "nvim-lua/plenary.nvim" from the two lazy.nvim install snippets in README.md (they list it under neotest's own dependencies).
4. Re-bump the pinned deps/neotest commit in Makefile/scripts/minimal_init.lua to one at or after neotest's plenary-free commit.
5. Verify: rm -rf deps && make install && make test-e2e passes on Ubuntu + Windows with no plenary anywhere.

Blocked on

nvim-neotest/neotest#611 — "plenary.nvim is going to be archived" (same root event as our #317: nvim-lua/plenary.nvim#677).

When that upstream issue is resolved (neotest removes its internal require("plenary.*")s), do the steps above and close this issue.