fix(commands): replace numbered placeholders in descending order

[OnlyYu1996]

Fixes PlatformNetwork/bounty-challenge#53172.

substitute_placeholders() now replaces numbered placeholders in descending order. That prevents $1 from rewriting the prefix of $10, $11, etc. before those larger placeholders are handled.

The existing "highest numbered placeholder captures the remaining arguments" behavior is preserved; regression coverage includes both direct $10 substitution and $10 as the rest-capturing placeholder.

Validation:

• cargo +1.90.0 fmt --package cortex-commands --check
• cargo +1.90.0 test -p cortex-commands test_substitute_two_digit_placeholder -- --nocapture
• cargo +1.90.0 test -p cortex-commands
• git diff --check

[greptile-apps]

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