* fix(process): use `./` prefix when sourcing env files in tmux for zsh

zsh's POSIX `.` builtin does not search the current directory unless
`.` is in `$PATH` — the default macOS shell is zsh, so the bare
`. '.env'` form silently failed there with "no such file or directory"
even after the cd-first fix from #18. Services in tmux windows ended
up without the per-slot env: `ECLUSE_*` vars from the per-slug preamble
were fine, but anything from `.env`, `.env.local`, or `.env.ecluse`
that wasn't duplicated into the preamble was lost.

build_source_preamble now emits `. './.env'` (and the same for
`.env.local`/`.env.ecluse`). The `./` prefix is POSIX-correct and works
in bash, zsh, and dash. Adds three regression tests covering the
correct prefix, the missing-files-skip behavior, and the empty-tree
case.

Fixes #27

* fix(env): strip outer dotenv quotes when parsing .env files

parse_env_file treated everything after `=` as the literal value,
which round-tripped `FOO="bar"` as the 5-character value `"bar"`
(quotes included) when the value got re-emitted in the per-slug
tmux preamble as `export FOO='"bar"'`. Downstream consumers that
check against a literal string (e.g. `z.literal("development")`)
would reject the quoted value, breaking any framework using
conventional dotenv `KEY="value"` style.

A matched outer pair of `"..."` or `'...'` is now stripped — the
standard dotenv convention where `FOO=bar` and `FOO="bar"` mean the
same thing. Unmatched quotes (`FOO="oops`), inner escaped quotes
(`MSG="hello \"world\""`), and unquoted values containing literal
quote chars or `=` (`URL=postgres://x:5433/db?a=b`) are preserved
verbatim, so URLs and query strings round-trip unchanged.

Adds seven regression tests covering double, single, unquoted,
unmatched, inner-escaped, empty-quoted, and bare-quote-char cases.

Fixes #28

* docs: changelog entries for #27 and #28