refactor(guides): move cycle-attachment content to inter-canister-calls

[marc0olo]

Summary

• Moves the ## Calling canisters that require cycles section out of the cycles-management guide and into inter-canister-calls as a new ## Calls with attached cycles section. Attaching cycles to a call is a call mechanic, not an operational management concern.
• Also moves the ### Accepting cycles in your canister subsection (which was under "Topping up canisters") to the same new section, since it is the callee-side of the same mechanic.
• Modernizes the moved Motoko examples from the deprecated Cycles.add<system>() imperative style to the current (with cycles = amount) parenthetical syntax, and updates the Rust examples from the low-level msg_cycles_add call to .with_cycles() on the Call builder — consistent with how the rest of inter-canister-calls.mdx is written.
• Extends the callee example to require a specific fee, trap if the attached amount falls short, and accept exactly that fee — excess is returned to the caller automatically. The original "accept all" pattern was a tip-jar, not representative of how real fee-charging canisters (like the XRC) work.
• Renames the callee subsection to ### Charging a cycle fee and the example function to compute() returning (), making clear that cycles are payment for work rather than the subject of the function.
• Fixes icp new proxy --template proxy → icp new proxy --subfolder proxy (no --template flag exists; verified against .sources/icp-cli/docs/reference/cli.md). The bug was present in the original section and carried over during the move.
• Leaves a precise cross-reference in cycles-management.md pointing to the new location.
• Updates four inbound links: concepts/cycles.md (×2), guides/chain-fusion/exchange-rates.mdx (×2).

Sync recommendation

hand-written

Closes #231

[github-actions]

🤖 Here's your preview: https://kwnd6-zaaaa-aaaam-ai7va-cai.icp0.io

[marc0olo]

Code review: one CLI command error found.

All Motoko and Rust examples are correct:

• (with cycles = amount) parenthetical syntax is confirmed correct per the Motoko compiler docs
• Cycles.accept<system>(available) with mo:core/Cycles and mo:core/Runtime imports is correct
• .with_cycles(u128) on the Call builder is confirmed in ic-cdk/src/call.rs
• msg_cycles_available() / msg_cycles_accept() (Rust callee) are correct

Fix needed: The icp new command has no --template flag. The proxy CLI block uses:

icp new proxy --template proxy

The correct flag is --subfolder:

icp new proxy --subfolder proxy

Verified against .sources/icp-cli/docs/reference/cli.md — icp new accepts --subfolder, --git, --path, --branch, --tag, etc. No --template flag exists. This bug was present in the original section in cycles-management.mdx and was carried over unchanged.

[marc0olo]

Feedback addressed:

• Fixed icp new proxy --template proxy → icp new proxy --subfolder proxy (verified against .sources/icp-cli/docs/reference/cli.md)
• Extended "Accepting cycles in the callee" to require a specific fee (100_000_000 as an example constant), trap if available < fee, and accept exactly fee rather than all available cycles — excess is returned to the caller automatically. This matches how real fee-charging canisters (like the XRC) work and avoids silently draining an overpaying caller.

[marc0olo]

Re-reviewed after the follow-up commit. All examples are correct:

CLI fix: --subfolder proxy is now in place. Verified against .sources/icp-cli/docs/reference/cli.md.

Improved callee pattern: the new fee-based accept is correct on both sides.

• Motoko: debug_show fee is a built-in keyword (confirmed in Motoko language manual); Cycles.accept<system>(fee) accepts exactly the fee amount; the Cycles module docs confirm "any remaining available amount is automatically refunded to the caller" on return.
• Rust: msg_cycles_accept(FEE) with FEE: u128 matches the max_amount: u128 signature in ic-cdk/src/api.rs; the "accept exactly the fee; excess is returned automatically" comment is accurate per IC protocol behavior.

All Motoko/Rust imports, anchor links, and the --id-only / --proxy CLI flags are correct. PR looks good.