feat(doctor): report a key an env file sets twice, and let the editor resolve it

[geodro]

A site showed postgres on every surface while the application ran on SQLite, and nothing anywhere said why. Its env file set DATABASE_URL twice: lerd read the first, the framework read the last, and both were behaving correctly.

Neither reading is wrong in general, which is why this reports rather than resolves. Symfony's dotenv parses into an array so a later assignment overwrites an earlier one and the last wins. Laravel loads through phpdotenv's immutable writer, which refuses to overwrite a name that is already set, so the first wins. lerd reads the first everywhere. A single rule cannot be right for both, and picking one silently is how a site ends up served from a database nobody chose.

The doctor names the key, how many times it is set, the value lerd uses and the value at the end of the file:

⚠ Env Keys
    DATABASE_URL is set 2 times (lerd reads "postgresql://...", the last is
    "sqlite:///%kernel.project_dir%/var/lerd_test.db") — in .env.local, and a
    framework reading the last value uses a different one than lerd does; keep
    the one you meant

Only dotenv files are ambiguous this way. A PHP file's duplicate has one answer the language decides, a second define being ignored and a later assignment replacing an earlier one, and lerd reads those the way PHP runs them.

The env editor shows the same conflict against the buffer rather than against disk, so it tracks edits, listing each occurrence with its line number and value. Keeping one drops the others, which leaves an ordinary unsaved change the user reviews and saves with the normal button, the way a staged missing key already works. Nothing is rewritten behind their back.

Driven against a real Symfony site carrying the duplicate, and against five other real sites to confirm none of them gains a finding.

Closes #1390