fix(extensions): revert ext:export option name change and fix bugs

[ajperel]

Description

This commit addresses issues in the ext:export command options and fixes a path resolution bug:

1. Reverts the --extension-instance option back to --instance (and removes the -e short flag).
2. Fixes a bug in option registration where extension-instance and kit-instance options were defined incorrectly as separate arguments to .option(), causing Commander to treat them as boolean flags and throw "Too many arguments" errors.
3. Fixes a typo in the kit instance option name from ---kit-instance to --kit-instance.
4. Simplifies the export mode check by removing the undocumented and unused kits mode.
5. Renames loop variables in kitExportTarget to kitInstanceId and configDirPath for better readability and to avoid shadowing.
6. Fixes a path resolution bug in fnHandler where joining projectDir and already-absolute configDir caused mkdirSync to fail. Ensured kitExportTarget always returns absolute paths and used path.resolve for directory creation.

Scenarios Tested

• Built the project successfully (npm run build).
• Verified --help output shows options correctly formatted and documented.
• Verified running ext:export with --mode functions --instance <id> parses correctly and does not throw "Too many arguments" errors.
• Verified running with multiple -k options (e.g. -k id -k id) parses correctly.
• Ran unit tests using npm run tests.
• Verified exporting extension config locally writes to the correct directory without unexpected path resolution errors.

Sample Commands

• firebase ext:export --mode functions --instance firestore-bigquery-export
• firebase ext:export --mode functions --instance firestore-bigquery-export --kit-instance firestore-bigquery-export

TAG=agy
CONV=4603ef45-7e00-4639-b91a-2e65853ba34d

Description

Scenarios Tested

Sample Commands

[wiz-9635d3485b]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
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IaC Misconfigurations	-
SAST Findings	9
Software Management Findings	-
Total	9

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[gemini-code-assist]

Code Review

This pull request reverts the --extension-instance option back to --instance in the ext:export command, removes the unused kits mode, and fixes path resolution bugs by using resolve instead of join. The review feedback suggests throwing a FirebaseError if a specified --kit-instance is not found in firebase.json to prevent silent fallbacks, and simplifying the directory creation logic by directly using the already-resolved writeLocation.configDir path.