Update README to match context-based API implementation

[Copilot]

The README documented an obsolete dmenv_init(buffer, size) API that no longer exists. The implementation uses a context-based approach with dmenv_create(parent).

Changes

• All code examples: Updated to use dmenv_create(NULL) instead of dmenv_init(buffer, size), added context parameters to all function calls, and added proper cleanup with dmenv_destroy(ctx)

• API Reference: Replaced initialization section with context management functions (dmenv_create, dmenv_destroy, dmenv_is_valid, dmenv_set_as_default, dmenv_get_default), added context parameters to all function signatures, documented integer operations (dmenv_seti/dmenv_geti)

• Overview sections: Updated Features, Overview, and "What is dmenv?" to describe context-based management and inheritance capabilities

Example

Before:

static char env_buffer[8192];
dmenv_init(env_buffer, 8192);
dmenv_set("APP_NAME", "MyApp");
const char* name = dmenv_get("APP_NAME");

After:

dmenv_ctx_t ctx = dmenv_create(NULL);
dmenv_set(ctx, "APP_NAME", "MyApp");
const char* name = dmenv_get(ctx, "APP_NAME");
dmenv_destroy(ctx);

Original prompt

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This section details on the original issue you should resolve

<issue_title>Zaktualizować readme</issue_title>
<issue_description>Readme nie pasuje do implementacji - np jest tam opis funkcji dmenv_init, której już nie ma - teraz jest ten dmenv_create. Trzeba zaktualizować readme</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Zaktualizować readme #2

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