docs: wrap agent-firewall inject-target placeholders in backticks

[nickvigilante]

What

The --session-id-inject-target flag help is surfaced verbatim in Coder's
generated CLI reference,
docs/reference/cli/agent-firewall.md.
Its <host> and <glob> placeholders were parsed as unknown HTML tags by the
docs site and dropped, so the format and example rendered as broken text
(readers saw Format: "domain= [path=]").

Change

Wrap the format template and example in backticks so they render as inline code
(angle brackets inside a code span are inert). This matches coder/coder's
documented
Placeholders in angle brackets
convention. The raw string literal is converted to a double-quoted string,
consistent with the neighboring option descriptions.

No behavior change; the terminal --help output only gains the backticks.

Before:

Format: "domain=<host> [path=<glob>]". Example: --session-id-inject-target "domain=prod.coder.com path=/api/v2/aibridge/*".

After:

Format: `domain=<host> [path=<glob>]`. Example: `--session-id-inject-target "domain=prod.coder.com path=/api/v2/aibridge/*"`.

Downstream

Once this is released, coder/coder bumps github.com/coder/boundary and
regenerates agent-firewall.md to pick it up.

_{Tracking (Coder internal): DOCS-583}

This PR was created with AI assistance (Coder Agents).

[linear-code]

DOCS-583