Slack: markdown_to_mrkdwn uses \x01/\x02 placeholders that could collide with agent output

[thepagent]

Parent: #382

Problem

markdown_to_mrkdwn() in slack.rs uses \x01 and \x02 control characters as intermediate placeholders when converting **bold** → *bold* (to avoid conflict with *italic* conversion).

If an agent ever outputs these control characters in its response, the conversion would produce corrupted output.

Suggested fix

Add a comment documenting the assumption, or strip \x01/\x02 from input before processing. Probability of collision is extremely low in practice.

Severity: very low — cosmetic/defensive.

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[thepagent]

Closing — low-severity Slack nit, not blocking 0.7.7 GA.