refactor: rename scl_lsp → tia_lsp (project umbrella for future STL/LAD/FBD)

Rename the project from scl_lsp to tia_lsp to future-proof for additional
TIA Portal languages (STL/AWL, LAD, FBD). The 'scl' language name is kept
as-is for the SCL filetype and tree-sitter parser; future languages get
their own names (stl, lad, fbd).

Project-wide changes:
- lua/scl_lsp/ → lua/tia_lsp/  (plugin entry point)
- lua/scl/     → lua/tia/      (language modules, shared across TIA langs)
- require('scl.*')    → require('tia.*')
- require('scl_lsp')  → require('tia_lsp')
- LSP client name:    'scl_lsp' → 'tia_lsp'
- Diagnostic source:  'scl_lsp' → 'tia_lsp'  (src/diagnostics.lua)
- package.json name:  'scl-language-server' → 'tia-lsp'

lua/tia_lsp/init.lua:
- Add vim.fn.exepath('tia-lsp') detection so the plugin prefers the
  mason-installed executable and falls back to { 'lua', server_path }
  for manual installs.
- Add opts.ts_parser_url to configure the tree-sitter parser source.
- Remove hardcoded ~/dev/scl_lsp paths in favor of ~/dev/tia-lsp and
  the new ts_parser_url option.

Cleanup:
- Delete lua/tia/init.lua (dead code, unreferenced duplicate of tia_lsp).
- Delete scl_lsp.sh (replaced by the mason-installed bin/tia-lsp wrapper).
- Update README.md and AGENTS.md to reflect the two-repo split
  (tia-lsp server + tia-lsp.nvim plugin) and document Mason installation.

Unchanged (intentional):
- grammar.js, src/grammar.json, src/parser.c: tree-sitter language name
  remains 'scl' (it's the language, not the project).
- Filetype patterns ('scl', 'udt', 'db') in autocmds.
- :SCL* command names (per-filetype prefix; future :STL* etc.).

Tests pass with the same pre-existing failures as before the rename
(formatter VAR_INPUT test, udt_parser line 199 const-variable bug,
plc_json reference-project-missing). No new failures introduced.

test/test_udt.lua

@@ -2,7 +2,7 @@ local test = dofile("test/test_harness.lua")
 
 package.path = package.path .. ";lua/?.lua"
 
-local udt_parser = require("scl.udt_parser")
+local udt_parser = require("tia.udt_parser")
 
 local function test_udt_parser_basic()
   local content = [[