feat: add base64 builtin command

[chaliy]

Description

Bashkit has no base64 builtin. The base64 command is used in real-world skills.sh scripts for encoding binary data (e.g. subscription GUIDs → base64url for Azure portal URLs).

Reproduction

# Encode string to base64
echo "hello world" | base64
# Error: base64: command not found

# Decode base64
echo "aGVsbG8gd29ybGQ=" | base64 -d
# Error: base64: command not found

# Real-world usage from microsoft-foundry/generate_deployment_url.sh:
GUID_HEX="d5320f9a73da4a74b63983efebc7bb6f"
echo "$GUID_HEX" | xxd -r -p | base64 | tr '+' '-' | tr '/' '_' | tr -d '='
# xxd works, but the pipeline breaks at `base64`

Source: skills.sh leaderboard

Skill: microsoft-foundry from microsoft/github-copilot-for-azure (~57K installs, 17 leaderboard entries)

Script: plugin/skills/microsoft-foundry/models/deploy-model/scripts/generate_deployment_url.sh line 79:

ENCODED_SUB=$(echo "$GUID_HEX" | xxd -r -p | base64 | tr '+' '-' | tr '/' '_' | tr -d '=')

Cross-ref: specs/015-skills-analysis.md — External binaries table lists base64 as MISSING.

Suggested scope

Standard base64 encode/decode:

• base64 (encode stdin to base64, write to stdout)
• base64 -d / base64 --decode (decode base64 stdin to binary stdout)
• base64 -w 0 / base64 --wrap=0 (disable line wrapping — default wraps at 76 chars)

Implementation notes

The base64 crate is already a dependency — used in curl.rs:292-293 for HTTP Basic Auth encoding. The builtin would reuse the same engine (base64::engine::general_purpose::STANDARD). Minimal new code needed.